ELECTRICAL LABORATORY NOTES & FORMS. ELEMENTARY AND ADVANCED. J. A. FLEMING, F.R.S. / LIBRARY UNIVERSITY OF CALIFORNIA. .l$9& Deceived ^Accessions, No. CVj.w No. Works by Dr. J. A. FLEMING, F.R.S. The Alternate=Current Transformer, In Theory and Practice. Vol. I. THEORY. Vol. II. PRACTICE. Short Lectures to Electrical Artisans. Electric Lamps and Electric Lighting. Electrical Laboratory Notes & Forms. All the above are very fully Illustrated. Electrical %abovaton> IKlotes an6 jfonne. ELEMENTARY SERIES. 1. THE EXPLORATION OF MAGNETIC FIELDS. 2. THE MAGNETIC FIELD OF A CIRCULAR CURRENT. 3. THE STANDARDIZATION OF A TANGENT GALVANOMETER BY THE WATER VOLTAMETER. 4. THK MEASUREMENT OF ELECTRICAL RESISTANCE BY THE DIVIDED WIRE BRIDGE. 5. THE CALIBRATION OF THE BALLISTIC GALVANOMETER. 6. THE DETERMINATION OF MAGNETIC FIELD STRENGTH. 7. EXPERIMENTS WITH STANDARD MAGNETIC FIELDS. 8. THE DETERMINATION OF THE INTERPOLAR FIELD OF AN ELECTROMAGNETIC WITH VARYING LENGTHS OF AIR GAP. 9. THE DETERMINATION OF RESISTANCE AND TEMPERATURE COEFFICIENTS WITH THE POST OFFICE PATTERN OF WHEATSTONE'S BRIDGE. 10. THE DETERMINATION OF ELECTROMOTIVE FORCE BY THE POTENTIOMETER. 11. THE DETERMINATION OF CURRENT STRENGTH BY THE POTENTIOMETER. 12. A COMPLETE TEST OF A PRIMARY BATTERY. 13. THE CALIBRATION OF A VOLTMETER BY THE POTENTIOMETER. 14. A PHOTOMETRIC EXAMINATION OF AN INCANDESCENT LAMP. 15. THE DETERMINATION OF THE ABSORPTIVE POWERS OF SEMI-TRANSPARENT SCREENS. 16. THE DETERMINATION OF THE REFLECTIVE POWERS OF VARIOUS SURFACES. 17. THE DETERMINATION OF THE ELECTRICAL EFFICIENCY OF A SMALL ELECTROMOTOR. 18. THE EFFICIENCY TEST OF A MOTOR. 19. THE EFFICIENCY TEST OF A MOTOR-DYNAMO. 20. TEST OF A GAS ENGINE AND DYNAMO PLANT. ADVANCED SERIES. 21. THE DETERMINATION OF THE SPECIFIC ELECTRICAL RESISTANCE OF A SAMPLE OF WIRE. 22. THE MEASUREMENT OF LOW RESISTANCES BY THE POTENTIOMETER. 23. THE MEASUREMENT OF ARMATURE RESISTANCES. 24. THE STANDARDIZATION OF AN AMPERE-METER BY COPPER DEPOSIT. 25. THE STANDARDIZATION OF A VOLTMETER BY THE POTENTIOMETER. 26. THE STANDARDIZATION OF AN AMMETER BY THE POTENTIOMETER. 27. THE DETERMINATION OF THE MAGNETIC PERMEABILITY OF A SAMPLE OF IRON. 28. THE STANDARDIZATION OF A HIGH TENSION VOLTMETER. 29. THE EXAMINATION OF AN ALTERNATE CURRENT AMMETER. 30. THE DELINEATION OF ALTERNATE CURRENT CURVES. 31. THK EFFICIENCY TEST OF A TRANSFORMER. 32. THE EFFICIENCY TEST OF AN ALTERNATOR. 33. THE PHOTOMETRIC EXAMINATION OF AN ARC LAMP. 34. THE MEASUREMENT OF INSULATION AND HIGH RESISTANCE. 35. THE COMPLETE EFFICIENCY TEST OF A SECONDARY BATTERY. 36. THE CALIBRATION OF ELECTRIC METERS. 37. THE DELINEATION OF HYSTERESIS CURVES OF IRON. 38. THE EXAMINATION OF A SAMPLE OF IRON FOR HYSTERESIS LOSS. 39. THE DETERMINATION OF THE CAPACITY OF A CONCENTRIC CABLE. 40. THE HOPKINSON TEST OF A PAIR OF DYNAMOS. noti-s are copyright, timl all riylits of reproduction are reserved. Particulars will be found overleaf. The Notes are arrangedby DR. J. A. FLEMING, of University College, London, and are published by " THE ELECTRICIAN " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ^ / UFI7BRSIT1 This Series of Electrical 'laboratory IRotes ano jforms is prepared for those who are practically engaged in the work of systematic Laboratory Teaching, and who know the difficulty of obtaining from the majority of Laboratory Students accurate records of their work. A large part of a Teacher's time and energy are taken up in repeating the same practical instructions to different Students, and in insisting on careful and systematic records being made of the observations taken. The object of laboratory teaching, so far as beginners are concerned, is not merely to carry out a particular set of experiments, but to train the Students in habits of accuracy of observation and in proper methods of reporting results. 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ALL DESCRIPTIONS OF PRINTING AND PUBLISHING WORK UNDERTAKEN. INSTRUCTIONS FOB THE USE OF THE APPENDED (Electrical |aboratorg, |jlote0 anb Jforms. HESE Laboratory Notes and Forms are intended for the use and assistance of Students and Demonstrators in Electrical Laboratories. They are not designed to supersede oral or text-book teaching, but to aid the Student in acquiring a habit of immediately and systematically recording the results of observations made in the Laboratory. With the object of obviating a constant repetition of instructions the first, two or three pages of each Form are occupied with a brief account of the experiment or measurement to be made, and with practical notes on the precautions necessary to be observed in carrying it out. The Student should read this part care- fully before beginning the experiment, and the Teacher may amplify it, as much as necessary, with verbal explanations. 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Where loose copies of the Notes and^gflgMLare used, it is best to keep them in a Portfolio or similar case. r' * *** ^ ELECTRICAL LABORATORY NOTES AND FORMS. . 1. ELEMENTARY. Name Date Exploration of flfeaonetic jftelbs. Tin- "/>/>ctratus required for these experiments is a circular coil of insulated wire and a magnetic compass box with a degree scale and short magnetic needle having a larger index needle attached to it. Tlie box is arranged to slide along a rod fixed perpendicularly to the plane of the coil, and so that the needle is always on the. axis of the coif. The magnetic needle should not be larger t/iu one-tenth the diameter of the coil. Tin' Student is recommended to xb'fc/i tin' arrangement of the apparatus. At every point in the neighbourhood of a magnet or conductor carrying a current there is a magnetic force, and the region within which this force manifests itself is called a magnetic field. At each point in a magnetic field the magnetic force has a certain direction and magnitude. If a very long magnetic needle is held with one pole at any point in a magnetic field and the other pole removed quite away from that point, the magnetic force tends to urge the pole held in that field in a certain direction, with a force which is proportional to the product of the strength of the pole and the strength of the field at that point. If a very small magnetic needle, pivoted at the centre, is held at any point in a field, two equal and opposite forces act upon its cuds. If I is the magnetic length of this needle, m the strength of each pole, and H the strength of the field, then, when the small needle is held so that its length or axis is- at right angles to the direction of the field, the couple or torque acting on it is equal to m / H. The product ml is called the moment of the magnet, and is represented by M. Hence, the couple in such a case is numerically equal to the product of the moment of the magnet and the strength of the field = M H. Hence, we may define a magnetic field of unit strength as a field in which a small magnet of unit moment experiences a couple of unit magnitude when held with its magnetic axis at right angles to the direction of the field. A small magnetic needle, therefore, sets itself, when free to move in a magnetic field, in a direction which indicates the direction of this field at that point. If the needle is forced round into a position at right angles to the direction of the field, then the couple required to hold the needle in this new position is a measure of the strength of the field at that point. If two magnetic fields are created in one region which are in directions at right angles to each other, and if a small magnet is held in this region, it will take up a direction which indicates the resultant field at that point. A magnetic field has, therefore, direction and magnitude, and may be indicated on a diagram by a straight line, like a force or a velocity. .Magnetic fields are combined and resolved in accordance with the rules fur the resolution and composition of forces or velocities. If H is the magnitude of one field (represented by a straight line), and F is the magnitude of another field at right angles to H, and if a small exploring needle is held in the combined field, the direction of the needle will be such that its axis makes an angle 6 with the direction of the field H, so that p , or F=Htan<9. H If, therefore, H has a constant value, and F is variable in magnitude, we can find the relative values of F at various points by observing the tangents of the angles of deviation of a small exploring magnet placed in that region at those points. Pass a constant electric current through the circular coil of wire, and at any point on the axis of this coil place the small compass needle. Turn the coil round until the direction of the axis of the coil is at right angles to the magnetic meridian at that point. Observe the angle of deviation e of the small magnetic needle when placed at different distances D from the centre of this coil. Find out from a table of tangents the tangents of these angles, and measure at the same time the distance from the centre of the coil to the centre of the small needle. Enter up your results in Table I. opposite. Plot a curve showing the decrease of magnetic force due to the oil at various points along the axis of the coil. If the field of the coil is very strong relatively to that of the earth, use a small controlling magnet to increase the strength of the constant field. At any point P taken on a line drawn through the centre of a short magnet of length / and at right angles to its axis the magnetic force at that point is parallel to the axis of the magnet, and has a value which varies inversely as the cube of the distance from either pole of the magnet. Draw a diagram of magnetic forces proving this as a consequence of the fact that the force at any point due to each pole varies as - , where m is the strength of the pole and r the distance of the point. Prove it experimentally by placing a short magnet with its axis at right angles to the magnetic meridian, and by holding a small -compass needle at various points on a meridian drawn through the centre of the short magnet. Observe the angles of deflection of the needle. Enter up your results in Table II. Show that the product of the cube of the distance of the centre of the needle from the centre of the magnet and the tangent of the angle of deflection is a constant quantity. Draw a curve representing this decreasing force at various points on the equatorial line of a short magnet. If N and S are the poles of the magnet (see diagram), and if N S = /, and if the point P is taken on a line P drawn through the centre of the magnet at right angles to N S, then (JVJ the force at P due to each pole of strength in is j, where r = N P or S P. From the similarity of the triangles P T E, N P S, it will be seen that the resultant P R of these two forces ~ has a magnitude ~ =-. ^, since PR: = I : r. Hence the re- r z r ? r r - r sultant force varies inversely as the cube of r, that is, nearly as the cube of the distance from the point P to the centre of the short magnet. THE EXPLORATION OF MAGNETIC FIELDS. TABLE I. Tlie Axial Field of a Circular Current. Observation No 6 = Deflection of small exploring Tan 6. D = Distance of centre of exploring needle H- Constant magnetic field due to F = Htan0. Magnetic force at distance D from needle. from centre of coil. earth or other magnets. centre of coil. THE EXPLORATION OF MAGNETIC FIELDS. TABLE II. The Equatorial Field of a Short Magnet. Observation No = Deflection of small exploring Tan 0. D = Distance of centre of exploring needle H = Constant magnetic field due to D 3 tan 0=^ H = ratio of moment of short needle. from centre of short magnet. earth. magnet to field of earth. 4 These Notes are copyr'njht, or to _5 square of radius of coil x ampere-turns x moment of magnet 8 cube of mean distance from magnet to circumference of circle Note that the product m I = M is called the moment of the magnet. (2) This couple tends to set the magnet with its length perpendicular to the plane of the circle. If another constant magnetic force of value H is made to act in a direction perpendicular to the axis of the circular coil, under these two forces, the small needle will take a position with its length inclined at an angle 6 to the plane of the circular coil, and the ratio of F to H is equal to the tangent of 0, or F = H tan 6. Equating the values for the magnetic force F, we have Hence, for a given circular coil and a constant current passed through it, the magnetic force at points on the axis varies inversely as the cube of the distance of the axial point from the mean circumference of the coil, and the magnetic force varies as the tangent of the angle of deflection of the small needle held at that point. Prove the above formula (3) by using the sliding magnetometer and coil, consisting of a circular coil of insulated wire and a small magnetic needle which can be placed at various points on the axis of the coil. Put the needle at various distances from the plane of the coil and observe the angles of deflection of the needle due to a constant current in the coil and a constant controlling magnetic force H. Try also different values of the current A, and use the formula (3) either to find the value of A, knowing H and the dimensions of the coil ; or to find H, knowing A and the value of the dimensions R, D and N, as given to you, for the particular coil you are using. Plot a curve showing the varying values of the magnetic force along the axis of a circular coil at points taken equidistantly along this axis. Enter up your observations in the form given on the opposite page. Attend to the following points : In setting up the magnetometer, place it so that the direction of the earth's horizontal magnetic field (equal to 0'18 of a unit) is in a direction parallel to the plane of the coil. See that the current in the leading-in wires or resistances does not directly disturb this needle. Prove this by short-circuiting the circular coil. Take care to keep the current vcry constant in the circular coil by using an adjustable resistance (carbon) and an ammeter in series with it. The constants for the coil you are using are as follows : R= c.m. N = THE MAGNETIC FIELD OF A CIRCULAR CURRENT. Observation Angle of deflection of needle. tan = Tangent of angle of deflection. D = Distance of needle from centre of coil. A = Ampere current in coil. H = Magnetic controlling force. y Calculated value of THE MAGNETIC FIELD OF A CIRCULAR CURRENT. Observation No = Angle of deflection of tan = Tangent of angle of D = Distance of needle from centre A = Ampere current in H = Magnetic controlling y= Calculated value of needle. deflection. of coil. coil. force. tan 2 fl(R 2 + D-') 3 . These Notes are copyright and all rights of reproduction are reserved. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by " The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet, Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. No. 3 ELEMENTARY. Date Stanbarbisation of a {Tangent (Galvanometer b\> the Mater Doltametev. 7'/i<' nj>pai-tns required for these experiments is a small tangent galvanometer, and a ivater voltameter (preferably Hofmanns form), in which the two gases are separately collected in two tubes connected by a cross tube and to a reserruir of dilute acid. Use 1:10 dilute sulphuric acid as the electrolyte. The >'//'// -,,t is recommended to sketch the arrangement of the apparatus. When an electric current is passed through an electrolyte, or electrically decomposable liquid contained in a voltameter, it extricates from it two ingredients, called the ions, and delivers these at the metallic or other plates by which the current leaves and enters the liquid, and which are called the electrodes. The metal in the solution, or the equivalent body, such as hydrogen, is delivered up at the negative electrode. The amount in grammes of this metal or ion so delivered up in one second is called its electro-chemical equivalent. The practical definition of a current of one ampere is that it is an unvarying current which deposits in one second '001118 gramme of silver from a dilute solution of nitrate of silver. A current of one ampere deposits in one hour at the negative electrode (when d through solutions of these salts) 4'025 grammes of silver, 1'178 grammes of copper, and '03738 gramme of hydrogen. These are called the electro-chemical equivalents per ampere-hour. It can be shown that '03738 gramme of hydrogen has a volume of very nearly 440 cubic centimetres (440'15 exactby) when measured at 15C. and at a barometric pressure of 760mm. If an electric current is sent through a dilute solution of sulphuric acid (1 vol. acid, 9 vols. water), and it liberates at the negative pole a volume of r cubic centimetres of hydrogen, measured at tU. and II mm. barometric pressure, in x seconds, we can calculate what this volume r would IT become at the standard pressure of 760mm. by multiplying i> by . Similarly, we 760 can reduce it to the standard temperature (15C.) by multiplying again by 273 + 15 273-*- t' In practice, the resistances P and Q are the two sections of a long fine wire stretched on a scale, and S is a plug resistance box. A battery of one or two cells is joined to the ends of the wire, and the rectangle is completed by joining the standard resistance S and the unknown resistance R as in the second diagram. The galvanometer is joined in between the junction of S and R and the sliding contact on the bridge wire. Having joined up the resistances as above, find the point of contact of the slider on the wire at which the galvanometer gives no deflection when the battery key is down. Attend to the following points : (i.) Put down the battery key first before making contact on the slide wire. (ii.) Press the slide wire key as lightly as possible so as not to nick the slide wire. (Hi.) Obtain the balance as quickly as possible, because the current heats the conductors and therefore alters the resistance to be measured. When balance is obtained, read off the length of the two sections of the divided wire on either side of the slider. These lengths are proportional to the resistances of the sections, which are the resistances P and Q. Note the standard resistance S used, and calculate the value of the unknown resistance R from the equation P and Q are called the ratio arms of the bridge. The accuracy of the measurement depends, therefore, on the uniformity of the slide wire. Test this by measuring a few known resistances. The most favourable arrangement for sensitiveness is when the standard resistance S has a value about equal to the resistance R to be measured. Enter up your results in the form given on next page. Note that the sign stands for ohms, and mm. for millimetres. In order to assist in finding approximately the position of balance on the slide wire it is convenient to shunt the galvanometer by a German silver wire, which is removed when the balance is nearly obtained. This shunt wire should have about one-tenth the resistance of the galvanometer itself. THE MEASUREMENT OF ELECTRICAL RESISTANCE BY THE DIVIDED WIRE BRIDGE. Observation Ratio arms of bridge in mm. Standard resistance in Calculated value of unknown resistance in No ohms. ohms. P = Q = = R = Description of vire or conductoi measured. THE MEASUREMENT OF ELECTRICAL RESISTANCE BY THE DIVIDED WIRE BRIDGE. Observation Ratio arms of bridge in mm. Standard resistance in Calculated value of unknown resistance in Description of No ohms. ohms. P = Q- s- R = These Notes are copyright, and all rights nf reproduction arc rcscn-id. They are arranged by D. J. A. FI.E.MIXI;, of University College, London, and are published by "The Klectrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. Name Date _ ZTbe Calibration of tbe Ballistic Galvanometer. The apparatus required for these experiments is a sensitive mirror ballistic galva- nometer of suspended coil or suspended needle type, a condenser of variable capacity, a few secondary cells, and a voltmeter to measure potential. A discharge key is also required. Tin' Student is recommended to sketch the arrangement of the apparatus and circuits. A galvanometer consists in general of a magnet and a coil of wire. The magnet may be fixed and the coil suspended, or the coil fixed and the magnet suspended. In the normal position the magnetic axis of the magnet is at right angles to the magnetic axis of the coil. When a current is passed through the coil, the coil and the magnet act on one another in such a manner that they tend to place their magnetic axes parallel to one another. This movement is resisted by another force, which is called the control. The control in the case of the movable magnet galvanometer is generally an external magnet creating a constant field. The control in the case of the movable coil galva- nometer is generally the torsion of a wire or other suspension. If means are taken to bring the movable part, whether coil or magnet, to rest when disturbed as quickly as possible, this arrangement is called a damped galvanometer. If the arrangements are such as to permit the movable part to execute vibrations when disturbed, which are as little damped or resisted as possible, then it is called a ballistic galvanometer. The use of a ballistic galvanometer is to measure the quantity of electricity. The use of a damped galvanometer is to measure steady current strength. When any heavy body is movable round an axis, the sum of the products obtained by multiplying the mass of each particle m of the body by the square of its distance r from the axis, or taking the sum 2 m r*, is called its moment of inertia round this axis, and this is denoted by the letter I. In the case of bodies swinging round an axis the following are important quantities : do The angular velocity = y the total resistance of the circuit, or by the quotient of the, rate of change of the total induction through the circuit at that instant by the total resistance of the circuit. If we divide the whole time during which the loop is being moved away from the field into little elements of time dt, during each of ( 2 ) which the current in the loop circuit is i, the sum of all the products of the quantities i and d t from the beginning to the end of the movement is the measure of the total quantity of electricity, Q, which has been set flowing through the loop circuit. Hence, the quotient obtained by dividing the total change in induction through the circuit by the total resistance of the circuit, is numerically equal to the total quantity of electricity which has flowed through the circuit, or, B = Q, orB=RQ, or HNA=RQ (1) Hence, to obtain the value of the field H at any point, we have to hold a loop of wire of N turns, each turn of the loop having an area A, at that point in such a manner that the lines of induction pass perpendicularly through the loop. We have then to connect this loop with a ballistic galvanometer, and measure the resistance E of the whole circuit. If, then, the loop is snatched away, the ballistic galvanometer needle will d make a sudden deflection or " throw " 6, and the value of C sin ^ is a measure of this total quantity of electricity which has been sent through the galvanometer. Hence e Q=C sin ^> where C is the ballistic constant of the galvanometer (see Elementary Form No. 5). We have then 4 R C sin (2) The value of sin- is easily found when we know the excursion x which the 2 spot of light makes (assuming a mirror galvanometer used) and the distance d of the IT fi scale from the mirror, for - = tan 2 d, and hence e, and therefore sin -, can be found. ct L Note the following points. The correctness of the above reasoning depends upon the assumption that the ballistic galvanometer has a needle with such a slow period of swing that the whole of the impulse on the needle is over and complete before the needle has time to move from its normal position. Hence the loop must be snatched iiway very quickly. The needle must be quite stationary before taking the "throw." Using a ballistic galvanometer, take such " throws " with a loop circuit held at various points in the field of a magnet, and obtain numbers which are proportional to the relative strengths of the field at those different points. Take as the unit field of comparison the strength at some fixed point. Explore thus the field at various points along the axis of a magnet, using the field strength right up against the pole as r\ a standard of comparison. Before entering the values of sin ^ in the table correct all the observed values of sin- by multiplication by the factor (l + o)' where A is the logarithmic decrement of the galvanometer. THE DETERMINATION OF MAGNETIC FIELD STRENGTH. Observation No X Excursion of spot of d Distance of scale X d Corrected value of sin (? B! = Corrected value of sin for Ratio of 8 81* light. from mirror. -8 Standard posi- tion of loop. THE DETERMINATION OF MAGNETIC FIELD STRENGTH. Observation No a: Excursion of spot of d Distance of scale 1C d tan 9 fi Corrected value of sin| Corrected value of sin^ for Ratio of 3 Si' light. from mirror. = S standard posi- tion of loop. i / i These Notes are copyright and all rights of reproduction are reserved. They are arranged by DR. J. A. FLKMIXO, of University College, London, and are published by "The Electrician" Printing and Publishing Company, Limited Salisbury Court, Fleet, Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. . 7. ELEMENTARY. Name Date with Stanbarb flfeaonetfc Tin' "/'jxirntu* required for the. following experiments is: A straight coil or long bobbin of insulated wire wound on a hollow pasteboard, glass, or wooden tube ; the total number of turns of the ivire being known, and the distance between the cheeks of the coil. Also a carefully- wound bobbin of ivire with one layer of windings, the total area included by all the windings being known. A sensitive mirror ballistic galvanometer, a, box of resistance coils, and an exploring coil or loop are needed. 7/n- Student is recommended to sketch on foolscap-sized paper the arrangement of the apparatus and the connections. The magnetic force or field strength F at a point in the neighbourhood of a very long straight wire carrying a current of A amperes is numerically equal to the quotient of one-fifth of the current by the distance d of the point from the wire in centimetres, or '-* <" This formula is true only if the return conductor is at a very great distance. If a i-irde of radius d is described round the wire, with its centre on the axis of the wire and its plane perpendicular to the wire, this circle is a line of force ; since the magnetic force at every point is in the direction of this circle, and has a magnitude equal to that given by equation (l). The length of this line is equal to 2-rrd. This line is called also a magnetic circuit. If we multiply together the length of this magnetic circuit, viz., 2-rrd, by the value of the magnetic force along that circuit, viz., - , we obtain a product, viz., A, 1 U t ' 10 which is called the magneto-motive force along that line. In any case, whatever may be the form of the conducting circuit, or of the magnetic circuit linked with it, we always have the following general relation, viz. : The magneto-motive force along a ) 4^- J The total ampere-current flowing through, magnetic circuit j 10 \ or linked with, that magnetic circuit. Apply this to a general case. Let a wooden ring, of which the mean diameter is laiui 1 compared with the diameter of its circular cross section, be wound closely over wit li insulated wire. Let there be N turns of wire. Let a current of A amperes be sent through the wire. This forms what is called a circular solenoid. The magnetic force in the axis of this solenoid is everywhere along the circular axis of the solenoid. Call ( 2 ) the length of this circular axis L, and the magnetic force F. The total current flowing through this magnetic circuit is NA, and, by the above rule, 10 Hence F= 4 - ^A- (2) 10 L or the magnetic force in the centre of the circular solenoid is numerically equal to 4<7T times the ampere-turns per unit of length of the solenoid. The above rule also holds good for a straight solenoid or bobbin of wire, provided its length is large compared with its diameter. Hence the magnetic force in the interior of a long bobbin of insulated wire is obtained by multiplying -. by the ampere-turns on the bobbin per unit of length. Hence a long bobbin of this kind provides us with a standard magnetic field when a known ampere-current is passed through it. The dimensions of the long bobbin of wire given to you are as follows : Length between the cheeks centimetres. Total number of turns of wire = Calculate by equation (2) the magnetic force in the centre of the bobbin when currents of 1, 2, 3, 4, and 5 amperes respectively are sent through the wire. Place the other or secondary bobbin of wire in this known field, and connect it through a plug resistance box with the ballistic galvanometer. Measure the whole resistance of the circuit composed of the galvanometer secondary coil and connections. Suddenly interrupt the current through the standard coil and note the " throw " of the ballistic galvanometer. Try this with a constant current passing through the standard coil and various resistances unplugged out of the resistance box. Prove that the r\ product of the sine of half the angle of throw of the galvanometer needle, or sin - and Zt the total resistance of the galvanometer circuit is a constant quantity. Correct all the observed values of sin~ by multiplying by (l + o) ; where \ is the logarithmic decre- ment of the galvanometer. (See Elementary Form No. 5.) Enter up your results in Table I. Next take the standard secondary coil, which consists of a single layer of fine silk-covered wire wound uniformly on a tube of known diameter, the total area included by all the wire windings being known. Place this secondary bobbin in the know r n field of the standard coil, and pass various currents through this latter. Observe the throw of the needle of the ballistic a galvanometer when this current is arrested, and prove that sin - varies as the field of Zi the standard coil when the resistance of the galvanometer circuit remains constant ; in e other words, prove that the quotient of sin - by the ampere-current of the standard coil is constant. Enter up your results in Table II. Note that in all these experiments either a suspended coil ballistic galvanometer must be used, or else the standard coil must be placed so far away from the galvano- meter as not to affect it magnetically. EXPERIMENTS WITH STANDARD MAGNETIC FIELDS. TABLK I. Observation No X 5* Corrected value of . e A = Current through standard Calculated and corrected value of sin - from scale. light. I coil. 2 T- These Notes are copyright, and all rights of reproduction are reserved. They are arranged by DR. J. A. FLEMING, of I'niversity College, London, and are published by "The Electrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. No. 8 ELEMENTARY. Date Determination of the Magnetic Jtelb in the Hit (Sap of an Electromaonet. 7'/M ,,/,i,,ir'ititx required for 1 1, c following experiments is a ballistic galvanometer /> ZTbe ^Determination of "{Resistance with tbc post (Mice pattern Mbeatstone Tin' f>iirti/n.i needed for these experiments is a Post Office pattern of " Wheatstone's Bridge" suspended coil galvanometer, a few cells of a dry battery, some <(//. of wire for experiment, a vessel of paraffin oil in ivhich to heat tin- coifs, nn tbe potentiometer. Tin- aiiparatus required for these experiments is a potentiometer, which may be either a simple divided ivire potentiometer or a more complete form such as " Cromp ton's " ; a sensitive suspended coil galvanometer, three large cells of a secondary battery, a couple of standard " Clark " cells, a variable resist- ance or rheostat, and some experimental cells to test. Student it recommended to sketch the arrangement of the apparatus and circuits. The potentiometer in its simplest form consists of a tine wire a 6 of platinoid or German silver (see diagram) stretched over a scale which is divided into 4,000 parts. This wire should have a resistance of 40 or 50 ohms. To the ends of this wire are attached three large secondary cells B, and these cells make a fall of potential down this wire of about six volts. One terminal of the galvano- meter G is attached to one end of this slide wire a, and the other end of the galvanometer is attached to that pole of a Clark standard cell Ck, which is of the same sign as the pole of the secondaiy battery attached to the end of the slide wire to which the other galvano- meter terminal is directly attached. Thft second terminal of the Clark cell is connected to a slider S moving over the slide wire, and by means of which a contact can be made at any point of the slide wire. The Clark cell is a standard of electromotive force, and if correctly set up has the following values for its electromotive force (E.M.F.) at various temperatures. E.M.F. Temp. E.M.F. Temp. E.M.F. Temp, volts. C. volts. C. volts. C. 1-444 6 1-436 13 T428 20 1-443 7 1-435 14 T427 21 1-442 8 1-434 15 T426 22 1-441 9 1-433 16 1'425 23 1-440 10 1-432 17 1'424 24 1-438 11 1-431 18 1-423 25 1-437 12 1-430 19 The potentiometer is used to compare the potential difference or electromotive force of any other cell C with that of a Clark cell. ( 2 ) In order to avoid calculations, it is desirable to employ three cells of a secondary battery in connection with the slide wire, and to obtain a potential difference (P.D.) at the ends of the slide wire which is exactly equal to four volts. This is achieved as follows : A resistance R, which must be continuously variable, and the most convenient form of which is Mr. Shelford Bidwell's rheostat or Lord Kelvin's revolving rheostat, is inserted in series with the slide wire. The slider on the slide wire is set at that division on the scale which corresponds to the value of the Clark cell for the tempera- ture of the day. Thus, if the temperature of the room is 18C. the slider S is set to make contact at division 1.431, which corresponds to 1'431 volts. The resistance R is then varied until the galvanometer indicates no current when the slide key is down. When this is the case, we know that the fall of potential down the slide wire must be 1'431 volts for 1,431 divisions, and, therefore 4'000 volts for 4,000 divisions. This being done, any other cell or battery of which the E.M.F. is less than four volts, can be substituted for the Clark cell and connected up in the same way. If the position of the slider at which the galvanometer shows no current is ascertained, we then know the fall of potential down the slide wire due to the secondary cells is given directly in volts by the scale reading, and hence the E.M.F. of the cell or battery becomes known. Thus, assuming the resistance to have been set so that the Clark cell when at 18C. balances at 1,431 on the slide wire and the other cell to be tested balances at 1,902 on the slide, this would indicate that the last cell has an E.M.F. of T902 volts. Note that a high resistance must always be inserted in the circuit of the galva- nometer, so that the Clark cell may not under any circumstances send any sensible current. The value of the Clark cell can only be assumed to remain constant if it is used merely as a standard of electromotive force. Take the precaution of checking the reading of the Clark cell continually during the progress of the experiments, and if the galvanometer gives any indication when the slider is made to touch at the scale reading corresponding to the Clark cell value when the Clark cell is connected on, then a small adjustment must be made of the variable resistance R to bring back the galvanometer to zero. It is not possible to get good readings unless the potentiometer, battery and galvanometer are well insulated by placing them on slips of ebonite or paraffined paper. Using the potentiometer in this way, determine the E.M.F. of a simple cell consisting of a carbon and a zinc plate placed in bichromate of potash solution, and determine the E.M.F. every five minutes for an hour or two. Determine also the E.M.F. at the terminals of various forms of single and two-fluid cells. Enter up your results in the Table on the opposite page. The accuracy of the measurements will depend on the uniformity of the slide wire, since the assumption is made that equal lengths of wire correspond to equal falls in potential. The wire can be tested by measuring the length of wire which corresponds to a fall of potential equal to a known fraction of a volt, when a perfectly constant current is kept flowing through the wire. The wire should be rejected if non-uniform. ( 3 ) DETERMINATION OF POTENTIAL DIFFERENCE BY THE POTENTIOMETER. Observation \o Tempera- ture of room. E.M.F. of Clark cell. Scale reading when Clark cell is in Scale reading when E.M.F. to be deter- mined is in Value of un- known E.M.F. or P.D. Time. Remarks. circuit. circuit. ( 4 ) DETERMINATION OF POTENTIAL DIFFERENCE BY THE POTENTIOMETER. Observation No Tempera- ture of room. E.M.F. of Clark cell. Scale reading when Clark cell is in Scale reading when E.M.F. to be deter- mined is in Value of un- cnown E.M.F. or P.D. Time. Remarks. circuit. circuit. These Notes are copyright, and all rights of reproduction are reterved. They are arranged by DR. J. A. FLEMING. of University College, London, and are published by " The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FOR3IS. No. 11. ELEMENTARY. Name Date ZTbe flfeeasurement of a Current b\> the potentiometer. Tin- apparatus required for these experiments is a potentiometer and the auxiliary apparatus as described in Elementary form 10. In addition to this a scries of low resistance standards of 1 ohm, O'l ohm and O'Ol ohm must be provided. These are best made of manganin strip to avoid temperature errors. Tin 1 Student is recommended to sketch the arrangement of the apparatus and circuits. In Elementary Form 10 the use of the potentiometer has been explained as a means of determining a potential difference by comparison with the terminal potential difference or E.M.F. of a Clark standard cell. If a steady unidirectional current flows through a resistance, such resistance having sufficient sectional .area and surface not to be sensibly heated by it, there is a fall of potential down this resistance, and this potential difference can be compared with the terminal potential difference of a Clark cell by means of the potentiometer. The condition of success is, however, that the total fall of volts down the resistance must be less than the fall which can be measured on the potentiometer, and which is generally less than two or four volts according as two or three secondary cells are used to work the potentiometer. Thus, if we have a resistance of O'Ol ohm, and we pass through this a current of 70 amperes, we have a fall of pressure of 07 volt down this resistance, or generally Drop in volts Value of resistance Value of current down resistance. in ohms. in amperes. Hence, since 0'7 volt can be measured on the potentiometer, we have only to divide the value of the observed drop in volts by the value of the resistance to get the value of the current. The accuracy with which the current can be measured is therefore dependent upon the accuracy < if the low resistance. This resistance must be so constructed that it does not sensibly change its value when the largest current which it is intended to carry is sent through it. A resistance of this kind, say O'l ohm, is conveniently constructed by taking ten platinoid wires, each of No. 16 or No. 18 gauge and of such length as to measure one ohm. These ten wires, have their ends soldered to two large copper terminals. We have then a resistance of one-tenth of an ohm. capable of g without sensible heating ten amperes. If any current up to about ten ( 2 ) amperes is sent through this resistance, and if potential wires are joined to the copper blocks and connected to the potentiometer, we can measure the fall in volts down this resistance. Then ten times the fall in volts is the value of the current in amperes passing through the resistance. In joining up the potential wires from the standard resistance to the potentiometer be careful to see that the right direction is given to the fall in potential. Thus, suppose the positive pole of the actuating secondary battery is joined to one end of the slide wire, we will call this A. Then to A must be joined, also through the galvanometer, the positive pole of the Clark cell and the potential wire which comes from the end of the resistance at which the current to be measured enters. In making use of this method to standardize an amperemeter, ampere balance or galvanometer, the instrument to be standardized is joined in series (see diagram) with an appropriate resistance K. That is to say. a resistance such that when the largest current to be measured is sent through it the fall in volts down this resistance is not more, say, than two volts. Hence a series of such resistances must generally be used. The potential wares from the ends of this resistance are joined in the right direction to the potentiometer, and the potentiometer is set to read directly by the Clark cell Ck as described in Elementary Form No. 10. Various currents are then passed through the standard resistance and instrument to be standardized, and simultaneous readings taken by two observers of the scale reading of the instrument and the potentiometer reading, and the value of the scale reading of the instrument thus becomes known. The current sent through the instrument to be standardized must be extremely steady. No good results can be obtained with a dynamo current. Nothing but the extremely steady current of a large secondary battery will give satisfactory results. In this case the instrument to be standardized must be joined up in series with an appropriate variable resistance to be able to vary the current sent through it and so obtain different scale readings. R MMMr- Calibrate in this way one or more ammeters and enter up your results in the annexed form. Make a curve of errors for each ammeter, plotting the error + or in terms of the scale reading. MKASURKMENT OF A CURRENT BY THE POTENTIOMETER. Observatioi No Tempera- ture C. Slide wire reading for Clark Slide wire reading for fall of volts down Standard resistance used. Value of the current Beading of the ammeter. Error of ammeter. cell. resistance. in amperes. - MEASUREMENT OF A CURRENT BY THE POTENTIOMETER. Observation No Tempera- ture C Slide wire reading for Clark Slide wire reading for fall of volts Standard resistance Value of the current Reading of the Error of cell. resistance. in amperes. i ' These Notei arc copyright, and all rights of reproduction are reserved. They are arranged by DR. J. A. FLEMING, of University College, London, and published by "The Electrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. . 12. ELEMENTARY. Name Date H Complete IReport on a primary Battery. ' apparatus required for these experiments is a potentiometer (Cromptoris ii.n'iiiit/i-tiK-nt of the instrument being preferable), a sensitive suspended coil galvanometer, standard " Clark " cell, secondary cells for the potentiometer, and a series of resistances, 10, 1 and O'l ohm, of manganin or platinoid. TJtc arrangement of the apparatus is the same as for testing voltage or current by the potentiometer. Student is recommended to sketch the arrangement oftJie apparatus and circuits. The potentiometer is by far the most convenient and accurate method of making a complete examination of a primary or secondary cell. The cell to be tested (in this case a primary battery) is examined as follows : The zinc plate or active plate in the cell is first weighed carefully. If the zinc has been recently amalgamated it should before weighing be carefully wiped with some cotton wool or clean waste to remove any loosely adhering mercury. Its weight in grammes is then taken on a chemical balance and noted. The cell is then set up. for example, it is a carbon-zinc cell with a single exciting fluid. The cell is set in action and a resistance of platinoid, r (see Diagram), say of one or two ohms, according to the current required, joined across the poles of the cell. The time of starting the current is noted. Two leading wires are joined to the terminals of the cell, c, and taken to the potentiometer, which is set up exactly as in the last exercise (see Elementary Forms Nos. 10 and 11) for measuring E.M.F. and current. The potential difference between the terminals of the cell to be tested, 011 closed circuit, is then measured and compared with that of a Clark cell, Ck (see Elementary Form No. 10), and the instant at which this measurement is made is noted. The platinoid resistance across the poles of the cell is next removed, and the potential difference between the terminals of the cell on open circuit is measured again, the time of making the measurement being again noted. Let the potential diH'rivncL' an volts or ( 2 ) parts of a volt which is found between the terminals of the cell when it is on open circuit be denoted by V, and the potential difference which is found between the same point.s when the poles of the cell are short-circuited by the resistance of R ohms be called v. V will always be greater than v. If the internal resistance of the cell which exists at the instant of making the measurement v be called r, then by Ohm's law we have v ET R + r (1) since the current which the cell is sending at that instant is equal to . From the above equation we have vR + t>r==VR, (V-v) R. or r = (2) This last equation gives us a value of the internal resistance of the cell at the instant when it is sending a current of - amperes. These measurements of V and v must be repeated at intervals of time (the time being noted by the watch) for some hours, or until the cell has run down. The zinc plate can also be withdrawn at intervals, washed with clean water, dried and weighed. The results have then to be plotted out in a series of curves as follows : Take a horizontal line on sectional paper to any suitable scale to represent time, and mark off intervals to represent hours and minutes. On this line erect at the appropriate places vertical lines to represent to some suitable scale (i.) The open circuit volts V of the cell, v (ii.) The ampere current -5- given by the cell, (Hi.) The internal resistance r of the cell calculated from equation (2). Then draw curves through the tops of all these perpendiculars, and obtain curve of E.M.F., current and internal resistance. Integrate the area, by any means, included between the current curve and the base line, limited by the two extreme ordinates, and obtain the whole ampere-hours output of the cell up to the end of each hour. If the zinc has been weighed each hour, the loss in weight in grammes of the zinc can be obtained up to the end of each hour the experiment has lasted, and the ampere-hour output of the cell also obtained from this curve. Knowing that the full theoretical value of the consumption of zinc per ampere-hour of output should be 1'213 grammes per ampere-hour (since this is the electro-chemical equivalent of zinc), we can find the ratio between the actual ampere-hour output of the cell per gramme of zinc dissolved and the theoretical value up to and at the end of each hour. This gives us what is usually called the efficiency of the battery. It will be found on plotting the efficiency in terms of the time that it gradually falls in value as the time of experiment increases. Taking the cell given to you, make in this way a full examination and report on it, and draw a complete chart of curves showing the variation with time of the E. M. F., current, and internal resistance of the cell and its efficiency. TEST OF A PRIMARY CELL WITH THE POTENTIOMETER. Temperature of Clark cell = t = Value of Clark cell E.M.F. External resistance used to close the circuit of cell tested = E = 'C. volts. ohms. Observation No Time. Potential difference at terminals of cell when Potential difference at terminals of cell when on Calculated internal resistance of the cell Weight of zinc rod or plate Efficiency of cell. on open circuit closed circuit TT JP. = V volts. = / volts. r= _*' R. V '. TEST OF A PRIMARY CELL WITH THE POTENTIOMETER. Observation No Time. Potential difference at terminals of cell when Potential difference at terminals of cell when on Calculated internal resistance of cell ^s f Weight of zinc rod or plate Efficiency of cell. on open circuit V volts. closed circuit = v volts. v - P r K. V w. . These Notes are copyright, and all rights of reproduction are reserved. They are arranged by DR. J. A. FLEMING of University College, London, and are published by "The Electrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. 13. ELEMENTARY. Name .............. Date Stanbavbisation of a IDoltmeter b\> the potentiometer, Tin' opjHirxfK* required for the following experiments is a potentiometer, a standard " Clark " cell, and the galvanometer and secondary battery and rheostat jor working the potentiometer as described in Elementary Form No. 10. In addition to these a divided resistance is required which must be capable of being placed safely and without sensible heating across the highest potential difference to be measured, and must be divisible into sections. Some voltmeters to check are also required. The Student is recommended to sketch the arrangement of apparatus and circuits. If a resistance composed of a great length of fine wire is placed across two points between which a constant difference of potential is maintained, there is a uniform fall of potential down this resistance. If this resistance is made in sections, the fall in potential down any section is to the fall in potential down the whole wire as the resistance of the section is to the resistance of the whole wire, lu order to secure this uniformity the wire must be so arranged that all parts of the wire become equally heated when a current is passed through it. A divided resistance of this kind, made in such sections that one-hundredth or one-fiftieth of the drop in potential down the whole wire can be taken, is required in potentiometer tests of high voltage. A resistance divided in this way into sections of 100 : 1, 50 : 1, 20 : 1, or 10 : 1, is often called a volt box. By means of a divided resistance of this kind, the potentiometer can be employed to compare high potentials such as 100 or 200 volts with the potential difference of the terminals of a Clark standard cell. In order to check a voltmeter by the potentiometer, we proceed as follows : Let us assume that a voltmeter has to be checked of which the scale is divided to read from 40 to 100 volts, and it is desired to compare the indications of this instrument with a Clark standard cell as a standard of reference. The divided resistance, divided in the ratio of 100:1, is joined across the terminals of the voltmeter, and the two together are placed across the terminals of an incandescent lamp, or other resistance of such a character that 100 volts can be safely maintained across the terminals. In scries with the lamp ( =3 ) is placed a variable resistance, and by changing the value of this resistance the potential difference of the terminals of the lamp can be varied from 40 to 100 volts. A pair of potential wires are then taken from the small section of the divided resistance and joined to the potentiometer in the right direction (see Elementary Form No. 11). Let us suppose in the first case that the resistance in series with the lamp is all cut out, and that the potential difference (P.D.) at the terminals of the lamp is 100 volts. Then the P.D. at the terminals of the voltmeter to be calibrated is also 100 volts; the P.D. at the terminals of the divided resistance is also 100 volts, and the P.D. at the ends of the hundredth part of the resistance is one volt. This last P.D. can be accurately compared on the potentiometer with the P.D. of the terminals of a Clark standard cell. If the ratio of the divided resistance is 100 : 1, then the P.D. across the terminals of the lamp, and therefore across the terminals of the voltmeter, is 100 times that of the P.D. at the ends of the small section of the divided resistance. By varying the resistance in series with the lamp various voltages can be put upon the terminals of the voltmeter, and the actual observed scale readings of the instrument be compared with the values of the same potential difference as given by the potentiometer in terms of the Clark cell. Suppose, for example, that the potentiometer is arranged as required for measur- ing potential difference, that there are 2,000 divisions on the slide wire, and that by secondary cells a potential difference of exactly 2 volts is maintained at the end of the slide wire, as determined in Elementary Form No. 10. Let the balance with the galvanometer be obtained when the potential wires from the ends of the small section of the divided resistance are connected to the beginning of the slide wire of the poten- tiometer and to a point on the slide wire indicated by 991 divisions on the scale. Then, if the potentiometer is properly adjusted, the true potential difference between the potential wires brought from the small section of the divided resistance is '991 volt. If the divided resistance is divided in the ratio of 100 : 1, the P.D. between the ends of the whole resistance is 99 '1 volts, and this is the true potential difference on the volt- meter terminals. If the scale reading of the voltmeter is, say, 102'5, this shows that the error of the voltmeter is +3 '4 volts, or that it reads 3' 4 volts too high. By per- forming a similar measurement all along the scale, a scale error can be found and a curve of errors be constructed. Check in this way the voltmeters given to you, and construct for each a curve of errors by setting off distances on a horizontal line to represent the scale readings of the voltmeter and vertical lines drawn upwards ( + ) or downwards ( ) to represent to an enlarged scale the error + or of the instru- ment at these points. Note the vertical and horizontal scales need not be the same. STANDARDIZATION OF A VOLTMETER POTENTIOMETER. Ratio of sections of divided resistance used = BY THE Observation No Tempera- ture of Clark Value of E.M.F. of Clark Slide wire reading of potentiometer when Clark cell Slide wire reading of potentiometer when the section of True P.D. at terminals of voltmeter Observed scale reading of Error of voltmeter. cell. cell. is connected on. divided resistance is connected on. calculated. voltmeter. - STANDARDIZATION OF A VOLTMETER BY THE POTENTIOMETER. Observation No Tempera- ture of Clark Value of E.M.F. of Clark Slide wire reading of potentiometer when Clark cell Slide wire reading of potentiometer when the section of True P.D. at terminals of voltmeter Observed scale reading of Error of voltmeter. cell. cell. is connected on. divided resistance is connected on. calculated. voltmeter. These Notes are copyright, and all rights of reproduction are reserved. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The Electrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. No. 14. -ELEMENTARY. Name Date H photometric Examination of an Jncanbescent Xamp. The apparatus required for these experiments is a photometric gallery or bench, on ivhich must be fixed (a) a standard of light, (b) a photometer, (c) an incandescent lamp to be examined. This last must have a, suitable voltmeter across its terminals and an ammeter in series with it to read volts and current. The testing current should be supplied by secondary batteries. No satisfactory work can be done off dynamo machines directly. T/te means of standardizing the voltmeter and ammeter, and also of varying the working pressure, must be at hand. The Student is recommended to sketch the arrangement of apparatus and circuits. An incandescent lamp may have electric currents of various strengths, up to a certain limit, passed through it, and under these circumstances it gives varying candle-power and exhibits varying differences of potential (P.D.) between its terminals. If then we observe : (a) The terminal volts V, or lamp P.D., (b) the current A in amperes through it, or lamp current, and (c) the candle-power, C.P., in any direction, we can derive (i.) The hot resistance R of the lamp in ohms, which is numerically equal to the quotient of V by A ; (ii.) The total power W in watts taken up in the lamp, since this is numerically equal to the product of A and V ; (Hi.) The watts per candle-power w obtained by dividing W by C.P. Hence if we measure and observe the quantities A, V, and C.P., we can calculate the quantities V W I-J W-AV, ..jW The lamp should be arranged as follows : A lamp socket should have two long double flexible conductors (commonly called flexible cord) attached to its terminals. Two of these wires should be covered with red cotton and two with black, to avoid confusion. One pair (red and black) are attached to a convenient voltmeter having the necessary scale range, and which must previously have been calibrated by reference to an absolute standard (see Elementary Form No. 10). The other two wires (red and black) are connected to a circuit of constant potential, say a 100-volt circuit, passing through an ammeter, also calibrated, on one side, and a variable resistance on the other. The incandescent lamp to be tested is then placed in the socket, and the resistance adjusted until the ammeter shows a current not exceeding that which the lamp will bear. The voltmeter indicates then the P.D. on the terminals of the lamp. Tliis socket should be arranged to slide on the photometer bench on a suitable stand. At one end of the photometer bench is placed a suitable standard of light. The worst standard is the legal standard or parliamentary candle. In some caaee a gas burner is used with a slit in front of the flame to adjust the liglit to a value of two ( 2 ) candles, and this slit is called a " Methven " slit. This arrangement is by no means satisfactory, as variations of gas pressure, atmospheric pressure, atmospheric purity, and other causes create very marked variations in the illuminating power of the gas. The best arrangement is to employ a standard incandescent lamp the bulb of which is very much larger than is usually the case. Such a lamp, if not used much, will not blacken or deteriorate very quickly. This lamp is employed, in the first place, to standardise a number of similar secondary standard lamps, and these are used as the actual working standards on the photometer. The secondary standard lamps are marked with the mean candle-power which they give in a certain direction when a certain current is passed through them. The secondary standard lamp having been fixed on the photometer bench and adjusted by a separate ammeter and resistance to its normal candle- power, the other, or lamp to be tested, can be compared with it by using a photometer. This may be either a simple grease-spot disc, or any of the varieties of Kitcliie wedge photometers, or more elaborate devices. Using the photometer, the Student has to practice the art of moving the standard lamp and lamp to be compared to such distances from the photometer that the two illuminated surfaces to be compared on the photometer are of equal brilliancy. This is comparatively easy when the lamps are in a similar state of incandescence, but it is much more difficult when one lamp is much brighter than the other. At the same instant that one observer takes the candle-power, two other observers must take the lamp volts and current. These observed values must be recorded in the proper columns in the Tables on the following pages 3 and 4. The resistance is theu varied so as to alter the candle- power of the lamp, and a series of observations taken of the volts, current and candle-power of the lamp all the way up from dull initial incandescence to the highest volts the lamp will bear. The corresponding values of R, W and w are then calculated. The candle-power should be observed in various directions, as follows : (i.) With the plane of the loop of filament parallel to the photometric disc, (ii.) With the plane perpendicular to it, (Hi.) With the plane at an angle of 45 to it. The mean value of these should be taken as the observed candle-power. Having obtained all these observed and calculated quantities, they should be set out in curves on squared paper as follows : Set off vertical distances to represent to some suitable scale mean candle-power, and prepare a series of four curves in which horizontal distances are respectively current, volts, watts, and watts per candle-power. Also prepare a fifth curve in which horizontal distances are volts, and vertical distances are hot resistances. Having obtained these curves, convert them into logarithmic curves by plotting horizontally and vertically the logarithms of amperes and candle-power, and the logarithms of volts and caudle-power. These last curves should be nearly straight lines. Express the value of the candle-power, C.P., in terms of the amperes, A, and volts, V, in the form of two equations C.P. = PA tJ (1) C.P. = P'V^ (2) where P, P', Q, Q' are some constants. Since from (l) we have log C,P. =log P + QlogA, and from (2) log C.P. = log F + Q' log V, it can be shown that the index Q or Q' is the tangent of the angle which the straight line obtained in plotting logarithms of amperes or volts and candle-power makes with the horizontal line, and the constants P and P' are the anti-logarithms of the intercepts on the vertical axis. Obtain in this way equations for the candle-power of the lamp you are usinij, and compare the calculated and observed values of the candle-powers at various voltages. ( 3 ) A PHOTOMETRIC EXAMINATION OF AN INCANDESCENT LAMP. Observation No Ampere currents A. Terminal voltage V. Mean candle- power Watts AV = W. Hot resistance V Watts per candle AV Name of lamp. C.P. A IT-"- - *. A PHOTOMETRIC EXAMINATION OF AN INCANDESCENT LAMP. Observation No Ampere currents A. Terminal voltage v Mean candle- power Watts AV = W. Hot resistance V Watts per candle A V Name of lamp C.P. A* w mw - ' : These Notes are copyright, and all rights of reproduction are reserved. They are arranged by DR. ,T. A. FLEMIXI;, of University College, London, and are published by "The Electrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. NTo. 15 ELEMENTARY. Nume Date Determination of the Hbsorptive powers of SemWTransparent Screens. Tic <'j'/>aratus required for these experiments is a photometric bench provided ivith a photometer, standard of light, incandescent lamps, and instruments for measuring currents and potential differences. Also various semi-opaque and translucent shades, glasses and screens. Tic Student is recommended to sketch the arrangement of the apparatus. In the practical use of electric lights, whether arc or incandescent, it is usual to employ various forms of semi-opaque or translucent globes, shades or screens to diffuse the light. The object of using these shades is to obtain a larger and less dazzling illuminating surface. These screens, however, cut off a considerable percentage of light. It is required to determine for each particular shade or globe what this percentage is. An incandescent lamp is placed on the photometer bench and adjusted carefully to its normal volts. The candle-power of the lamp is then carefully taken in various posi- tions. If the lamp is placed vertically, and the vertical line through the centre of the lamp is called the axis, the candle-power should be taken in the direction of the axis, and in three directions horizontally, one with the plane of the filament parallel to the photometric disc, one with plane perpendicular, and the other with the plane at 45. The current through the lamp and the terminal volts should be kept constant, and be taken at the same time. When this is done, a shade, or globe, or screen is put over the lamp, which shade may be any of the ground glass, semi-opaque, or fancy shades sold for the purpose, and the candle-power taken at the same volts and current again. The difference of the candle-power in any direction with the globe on, and the candle- power in that direction with the globe off, gives the loss in candle-power due to the globe or shade, and this may be expressed as a percentage of the original candle- power in that direction. Using various screens, such as ground glass, semi-opaque glass, tissue paper, tracing cloth, writing paper, frosted glass, determine the per- centage loss of light due to these screens when placed over a 16-candle-power incandescent lamp, and record the results in the Tables provided on pages 3 and 4. ( 2 ) Note the following precautions in using the photometer : The eye is not in a suitable condition for discriminating small difference of brightness of two surfaces if it has been recently stimulated with bright light. It is therefore impossible to make good photometer measurements unless the eyes have been preserved in dim light for some time. Photometer measurements should always be made, therefore, in a large well-ventilated photometric room or gallery, the walls of which are painted dead black, and in which no lights are used except those being compared. The observers must keep in this room some time before making the measurements. The eyes of different observers differ greatly in sensibility. Each observation should therefore be taken several times by different observers and the names of the observers recorded. It is impossible to get good results when candles or gas flames are used as standards of light in badly ventilated photometric rooms, as the supply of oxygen for the flame is insufficient. In making a comparison of the illuminating power of one light with that of a standard, when once the approximate distances have been discovered at which the lights balance on the photometer, the measured light or the photometric disc should be oscillated to and fro in gradually diminishing arcs until an exact balance is found. It is found that this process assists the eye in discriminating between a small difference in brightness of the illuminated surfaces compared and any small difference in colour which may exist. The student should therefore practise comparing one incandescent lamp at normal brightness with one of which the filament is kept at a red heat, and endeavour to overcome the difficulties attending the photometry of lights of different colour. ( 3 ) THE DETERMINATION OF THE ABSORPTIVE POWERS OF SEMI-TRANSPARENT SCREENS. Observation No Distance of standard lamp from Illuminating power of the standard Distance of in- candescent lamp from Illuminating power of the incandescent Illuminating power of the incandescent lamp Nature of screen Percentage absorption of screen photometric disc. lamp. photometric disc. lamp, niiroreretl. covered with srrefn. THE DETERMINATION OF THE ABSORPTIVE POWERS OF SEMI-TRANSPARENT SCREENS. Observation No Distance of standard lamp from Illuminating power of the standard Distance of in- candescent lamp from Illuminating power of the incandescen Illuminating power of the incandescent lamp Nature of j Percentage absorption jhotometric disc lamp. photometric disc. lamp, uncovered. caveivil witli screen. . ' These Notes are copyright, awl all rights of reproduction are reserved. They are arranged by DR. J. A. FLEMING. of University College, London, and are published by " The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street. London, Kngland. ELECTRICAL LABORATORY NOTES AND FOR1IS. No. 16. ELEMENTARY. Name ^Determination of the IReflective power of Darious Surfaces. The apparatus required for these experiments is a photometric bench fated up as described in Elementary Form No. 14 for the photometric measurement of incandescent lamps, and provided with a standard of light. The actual means of comparison, tvhether Bunsen grease-spot disc, wedge, or utht'r photometer, must be enclosed in a box provided ivith tivo small round < 1 1 >c rturen on either side, and be so placed that rays from the. incandescent lamp being measured can only fall on the disc when sent along the direction of the photometer bench. The walls of the photometer room or gallery should be painted dead black, or be lined with black velveteen. The. Student is recommended to sketch the arrangement of apparatus and circuits. When light from any source falls on a polished or reflecting surface, some of the light is reflected regularly according to the laws of reflection. The percentage value, expressing the ratio of the intensity of the reflected ray to the incident ray, is called the coefficient of reflection. Some of the light is also irregularly reflected or scattered. The value of the coefficient of reflection depends upon . the angle of incidence of the ray. Nearly all surfaces reflect a much larger proportion of the incident light at large angles of incidence than at small angles. In order to determine the coefficient of reflection of a plane mirror or looking- glass at various angles of incidence the following procedure is adopted : A standard incandescent lamp is placed as a standard of light on the photometer bench, and means are taken, by the employment of a resistance, to keep the current through it perfectly constant during the experiments. Another incandescent lamp is placed on the other side of the photometer disc, and in the same way its illuminating power is preserved constant by keeping a constant and regulated current passing through it. The most convenient way of doing this is by means of a carbon resistance, which consists of plates of carbon more or less compressed with a screw. Thus can very fine adjustments of resistance be made, and the current in a circuit be governed with great exactness. The second incandescent lamp being placed on the photometer bench is first photometered directly against the standard in some definite position ; say, with the plane of the looped filament parallel to the plane of the photometric disc. A glass mirror or looking-glass, or silvered mirrors of about 6 inches by 4 inches in size, is then placed on the-photometer bench, with its plane at any angle to the axis of the bench, and the incandescent lamp is moved into such a position that its light cannot fall on the photometric disc, except after reflection at this angle in the mirror. The incandescent lamp is moved to such a distance that the balance of illumination is obtained on the photometric disc, between the light of the standard lamp and that of the incandescent lamp after reflection at the desired angle in the mirror The distance of the incandescent lamp from the photometer disc is then measured. The illuminating power of the incandescent lamp is to that of the standard lamp as the square of the distance of the incandescent lamp from the disc is to the square of the distance of the standard from the disc. The coefficient of reflection of the mirror at the determined angle is then obtained by expressing as a percentage the rat id of the candle-power of the incandescent lamp, measured after reflection in the mirror to the value measured directly without reflection. Thus, if the value of the standard lamp is 16 candles, and the incandescent lamp is found also to have an illuminating power of 16 candles when measured directly, but only of 14 candle- power when the ray is measured after reflection at an angle of 45 from a plane mirror, 14 the coefficient of reflection is jp = 87 per cent. In the same way the coefficient of reflection may be determined at various angles and for various surfaces. The chief precaution w r hich must be taken is to test whether any light from the incandescent lamp when in the second position can fall on the photometric disc when the mirror or reflecting surface is taken aw r ay. If this is found to be the case, scattered light is getting into the photometer box, and this must be prevented by screens of black velvet suitably placed. Measure in this way at various angles of incidence the coefficient of reflection of a plane mirror or looking glass, of polished metal plates, white paper, brown paper, and painted wood, and enter up the results in the annexed tallies. REFLECTIVE POWER OF VARIOUS SURFACES. Observation No Candle- power of standard lamp Distance ; of standard from the photometric Distance of incandescent lamp from the photometric Illuminating power or candle-power of incandescent lamp Illuminating power or candle-power of incandescent lamp Angle of reflection = 0. Coefficient of reflection disc = ,/,. disc measured directly ""'' measured after reflection = !,. i "I," REFLECTIVE POWER OF VARIOUS SURFACES. Observation No. Candle- power of standard Distance of standard from the photometric Distance of incandescent lamp from the Illuminating power or candle-power of incandescent lamp Illuminating power or candle-power of incandescent Angle of reflection -6 Coefficient of reflection = 1. disc -d,. disc -d measured directly =i,.i(|)-. measured after reflection = !,. I* li' These Notes arc copyright, and all rights of reproduction arc resened. They are arranged by DB. J. A. FLEMING of University College, London, and are published by "The Klectrician" Printing and Publishing Company, Limited. .Salisbury Court, Fleet Street, London, Kngland. ELECTRICAL LABORATORY NOTES AND FORMS. No. IT. ELEMENTARY. N the Crable flftetbofc. nf>jtr"tus required for this exper'mwnt is a small electromotor, which may lie shunt or seines motor. The motor is balanced oil a " Bracketl " i-i-iii(te. A /F. Hence the efficiency e of the motor is given by e = AV .w W (2) The most simple method of determining the torque or couple F in a running motor is by means of a "Brackett's" cradle. The motor is fixed on a small wooden cradle- balanced on knife edges, and of such size that the line through the knife edges passes through the axis of the motor. To this cradle is attached a long arm and sliding weight. The motor must be screwed to the cradle, and by means of counterpoise weights the centre of gravity of the mass must be brought up or down it so as to be on the axis line through the knife edges. The cradle is then balanced with its knife edges on steel planes carried on a suitable frame. A cord is then wrapped once round the motor pulley and stretched at both ends, so that, when pulled in opposite directions, friction is put upon the pulley when current is put into the motor, and this cord brake is applied to make the motor give out, work. The armature of the motor, by its reaction on the lields, tends to tilt the fields, and therefore the whole motor, on its cradle in an opposite direction. The motor can be restored to its initial position by sliding a weight on the balance arm. AY hen this is done, the product of this weight P and the distance at which it is placed from the axis gives us the couple P required to keep the field magnets from turning round, and therefore represents the couple which the armature is exerting. Hence P a, so measured in feet and pounds, or centimetres and grammes, gives us the value of the motor torque or couple F. The speed of the motor is at the same time observed, and also the value of the current A and the terminal volts A". Carry out in this manner a series of experiments with a small motor, and determine its efficiency at various speeds and loads, from the equation (2) above. NOTE. 4 horse power = 746 watts = 550 foot pounds per second. Hence, if the torque F is measured in feet-pounds, w F must lie reduced to watts by proper multiplication by 746 and division by 550. EFFICIENCY OF A SMALL MOTOR BY THE CRADLE METHOD. Observation No. . Speed N. Current in amperes = A. Terminal P.D. in volts = V. Balance weight in His. = P. Length of arm in feet Power put in in watts = W. Power taken out in watts = *. Efficiency of motor EFFICIENCY OF A SMALL MOTOR BY THE CRADLE METHOD. Observation No Speed N. Current in amperes Terminal P.D. in Balance weight in Ibs. Length of arm in feet Power put in in watts Power taken out in watts Efficiency of motor = A. = V. = P. = . = W. = if. - These Notes are copyright, and all rights of reproduction are reserved. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The Electrician" Printing and Publishing Company, Limited Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. No. 18. ELEMENTARY. \ tbe Brake Tin- i//>in-"t 2ir n, where n revolutions per second), the external rate of doing work is U T. Hence, adding all these modes of consumption of the external power applied together, we have W = A V = av + a*r + d +/+ 'j>/)aratus required for this test is a gas engine and dynamo. The gas engine must be provided ivith an indicator and indicating gear, and with a counter for taking the number of explosions and the speed. It is desirable also to measure the water passing through the cylinder jacket, and the temperature of the incoming and outgoing water. The electrical poiver given out by the dynamo should be taken up in resistances. In default of anything better, an iron wire resistance may be used, but a bare platinoid wire immersed in running water forms a better absorber of power. Tin' Students assisting at this test should all make sketches and diagrams of the circuits and connections. In taking a complete test of a motive-power or electrical power plant of any kind, we wish to discover all the ways in which the energy supplied to the plant is transformed and used, and the relative amounts of the same. For this purpose there is no better plan than to make a kind of balance sheet of energy in which we put on the debtor side all the energy given to the plant, and on the creditor side all the items of energy given out by it. It will be assumed that the gas engine used in this test is the ordinary double-cycle gas engine, and that the dynamo is the ordinary shunt wound dynamo as used for accumulator charging, since this plant is most frequently found in electrical laboratories. The quantities which have to be measured and recorded are as follows (one observer should be delegated specially to record each item during the run) : 1. The volume of gas used by the engine = G. This must be recorded by a meter on the intake gas pipe of the engine. If there is no special meter here the test should be made at a time when all other gas lights in the building can be put out and the record taken on the main meter, but this is not so satisfactory as a special meter. 2. The amount of cooling water used in the cylinder jacket = W. Whenever possible it is desirable to have a water meter put on a direct supply of water to the jacket and to take the temperature of the incoming and outgoing water. This, combined with the known volume of water used, will give the thermal units removed from the cylinder during the trial. 3. If possible the oil used during the trial should be noted. See that all bearings and lubricators are in good order and filled up before starting. 4. The number of revolutions of the engine per minute = N must be taken. 5. The number of explosions per minute = n must be counted. This can be taken by an automatic counter, which is moved by the inlet gas valve lever at each admission of gas to the cylinder. 6. The indicator diagrams must be taken. The ordinary Richards High-Speed Indicator may be used, or else the newer form by Wayne. Cards should be taken at regular intervals during the run, and the I.H.P. worked out at once. 7. The ampere-current = A given out by the dynamo must lie noted at intervals, the time being taken when the observation is made. 8. The volts = V at the terminals of the dynamo are to be recorded at the same instant that the current is measured. The product of A and V gives the power in watts being given out by the dynamo. The mean value of A V during the run gives the mean power in watts so A V generated. The quotient of ' ; gives the same in horse-power, and the product 746 of this last and the time of the run in hours gives us the energy in horse-power hours given out by the dynamo in the run. One Board of Trade unit = 1,000 watt-hours = ( = 1'3 nearly) horse- 746 power hours of energy. The above eight quantities should be simultaneously observed during a run of two or three hours, and each observer should afterwards get from the others all the observed values and enter them up on the Tables appended. It is a good plan to appoint one observer as fugleman, who gives the word when the observation is to be taken, and notes by a watch the instant when it is so done. In this way a regular series of observations at intervals of about 10 minutes or so can be obtained, and the readings of all the instruments are simultaneous. The observations are then to be reduced as follows : From the indicator diagrams the mean pressure = P during the explosion must be obtained. This can be done best by the planimeter (Amsler's), taking the whole area and dividing it by the length of the diagram. Knowing the spring and scale, we obtain the mean pressure in pounds per square inch. The piston area = a and stroke = s must be measured in feet and square inches, and the indicated horse-power = H. P. calculated from the formula TTrp _aPsn 33,000' where n is the number of explosions per minute. We have then a record of the I.H.P. at various intervals of time. If these are equidistant we can obtain easily, by taking the mean of them, the mean I.H.P. during the run. From the observed volume of gas = G used we can obtain the cubic feet of gas per I.H.P., and from the water used the gallons of cooling water circulated per I.H.P. The mean electrical output of the dynamo in horse-power is also obtained by multiplying the mean value of the product AV and 746. This is sometimes called the electrical horse-power of the dynamo, and is denoted by E.H.P., just as the indicated horse-power of the engine is denoted by I.H.P. The ratio of E.H.P. to I.H.P. is called the combined efficiency of the engine and dynamo. The gas and water used can also be reckoned out in terms of the electrical horse-power. The following are the data of the engine and dynamo used in this trial : GAS ENGINE No. DYNAMO No. Makers, Messrs. Stroke = s inches. square inches, revs, per min. Piston area a = Normal speed = Type of engine = Resistance of fields Resistance of armature Current in fields Makers, Messrs. Normal speed Normal volts Full current Type of dynamo revs, per mm. volts. amperes. GAS ENGINE AM) DYNAMO TRIAL. Made at. Date TABLK I. Time of observation Current from dynamo = A. Volts at dynamo terminals -V. Number of explosions in one minute = 11. Gas meter reading = G. Water meter reading = if. I.H.P. from card taken during the minute. Efficiency E.H.P. ~LH7PT - -* * GAS ENGINE AND DYNAMO TRIAL. Made at.. Date TABLE I. Time of observation. Current from dynamo = A. Volts at dynamo terminals -V. Number of explosions in one . minute = n. Gas meter reading = G. Water meter reading = . I.H.P. from card taken during the minute. Efficiency E.H.P. I.H.P. - 1 TABLE II. Mean I.H.P. during run. Mean E.H.P. during run. Total gas used during run. Total water circulated during run. Cubic feet of gas per mean I.H.P. Gallons of water per mean I.H.P. Cubic feet of gas per mean E.H.P. Gallons of water circulated per mean E.H.P. These Notes arc copyright, and all riijhU of reproduction are reserved. They are arranged by DR. J. A. FLEMING, of University College, London, and are j ublished by "The Electrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. of rax TJHIVSRSIT7 ELECTRICAL LABORATORY NOTES AND FORMS. . 21.- APVAWCEP (No. 1). Xxme Date Determination of tbe Specific Electrical IRestetance of a Sample of metallic Mire. '/'//<' iijtpnratuif required for this determination is a Wheatstone's Bridge set, coupled, in't/i " M'ltxih'rc movable coil galvanometer. The coils of the bridge should previously have been tested and compared with a standard coil or coils, of ir/iich the true electrical resistance is known at definite temperatures. For tin' measurement of specific gravity a good chemical balance and weights are n-ifiu'red. For the measurement of length a metre metal scale and beam compass are needful. Tl' S/H, /,',,t is recommended to make sketches of the arrangement of the apparatus, and to draw carefully on squared paper the curves representing the observation. The specific electric;! 1 resistance of any material is the resistance of a mass of known volume or weight of it in a certain form and taken between defined surfaces. By volume specific resistance is meant the electrical resistance of one cubic centimetre of the material measured between opposed surfaces of the cube and expressed in abso- lute l.'.G.S. units. By mass specific resistance is meant the resistance of a known mass of the material, viz., one gramme, in the form of a circular-sectioned wire one metre in length, the resistance being taken between the ends of this wire. If the resistance of a uniform circular-sectioned wire of length I centimetres and diameter d centimetres is equal to R ohms, then, if p is the volume specific resistance as above defined, we have 10 9 ,rd 2 R /,, ' = 4/ ' ' ' () multiplier 10 9 is required to reduce resistance measured in ohms to its value in ( '.( !.S. absolute units. If w is the weight in grammes of this wire and s is its specific gravity, then , 4 Hence, by substitution from (2) or (l) we get 10 9 Rw ,o\ and f> thus becomes known when we know the resistance in ohms of a length / centi- metres of the wire, its weight w in grammes, and its specific gravity *. The determina- tion of the volume specific resistance requires, therefore, the determination of the length, weight. specific gravity and electrical resistance of a piece of circular-sectioned wire drawn from the material. In the case of most ordinary metals, the most convenient diameter for the wire to be used is about 0'02 of an inch, or 0'5 millimetre, and the length may be from one to six metres. Let us assume that the volume specific resistance of a sample of copper wire is to lie determined. The wire having been carefully drawn down to about the above dia- meter, the operations are conducted in the following order: On a stout plank of deal small brass plates are screwed, with centres at distances of one metre apart. Transverse scratches are made on these plates by means of beam compasses, such scratches being exactly one metre apart. Two lengths of the wire to be measured are then laid over the board and carefullystraightened,and are each cut two centimetres longer than three metres. These two wires must then be covered with cotton, or silk wound on, to insulate them. A terminal rod of high conductivity copper of about O'l inch diameter and 1 foot in length is then carefully soldered to one end of each of the two lengths of insu- lated wire, the overlap of wire against the rod being exactly one centimetre. The other ends of the two lengths are carefully twisted together ; one centimetre of the length, exactly, being employed for the twist, and this twist is over-wound with copper binding wire and well soldered. The doubled and insulated wire is then coiled up carefully into a flat circular coil of about six inches in diameter, and the stout terminal wires bent up at right angles. If the wire is springy it may be tied with tape to keep it in the requisite form. We have then a coil of wire wound non-inductively and exactly six metres in length. The next step is to take the electrical resistance of this wire at zero centigrade. For this purpose the flat coil is placed in a circular glass beaker of size large enough to hold it, and the beaker is filled with paraffin oil. The beaker must l>e placed in a wooden tub and surrounded with broken ice. The paraffin is then kept well stirred and its tempera- ture taken with a corrected thermometer. When the temperature is stationary the resistance of the coil is taken on the bridge. The ice is then replaced by water at different temperatures, and the resistance is again taken in like manner at the various temperatures. The paraffin oil must be kept well stirred and the temperature main- tained as steadily as possible during each set of observations. These observations should be repeated several times, so as to obtain the electrical resistance of the coil at various temperatures between 0C. and 100C. A correction has to be applied to these observed resistance values for the resistance of the thick copper connectors. This must be ascertained by measuring on the bridge the resistance of a loop of the same thick copper wire as that used for the connectors, and equal to them in length taken together. These resistance measurements being corrected, we have the resistance at various temperatures of a known length of the wire. The wire is then cut away from its connectors, and the silk or cotton insula- tion carefully removed and the wire cleaned with ether and alcohol with as little rubbing as possible. The lengths are then remeasured and are folded up as compactly as possible and weighed on the . chemical balance. After weighing in air, the lengths are weighed hanging in distilled water. To get rid of the air which clings to the wire it must be gently warmed in boiled distilled water, allowed to cool under the water, and then hung by a very fine platinum wire to one scale pan, and the nett weight of the wire taken when hanging in distilled water at 15C. If W is the \veight in air and W 1 the weight in water, all corrections being applied, then the specific gravity s is given by the equation W W-W 1 ' If very exact results are required, a correction must be applied for the change of density of the distilled water with temperature, but the other errors of observation will generally mask this correction. One point to notice is that, in weighing the wire in water, it should be suspended by a fine platinum wire of about - 004 inch in diameter rather than by a thread. The weight of this platinum wire alone, as immersed in the water, must be taken and deducted from the gross weight. The difficulty of making a very exact weighing of a body hung in a liquid is that the capillary action of the sur- face layer of the liquid resists the movement of the suspending fibre through it, and thus renders the balance less sensitive. For precautions in the use of the balance in weighing and in taking the specific gravity or density of the material, the student should consult any of the following text- books on Practical Physics: Glazebrook and Shaw's "Practical Physics," Chapter Y. : Kohlrausch's "Physical Measurements"; or Nichol's " Laboratory Manual of Physics," Vol. I., Chap. II. ' The length, resistance and specific gravity being carefully determined, we can insert the observed values in the formula (3), and obtain the value of the specific volume resistance p of the material. From the- values of the resistance at various temperatures the volume specific resistance can be calculated for these different temperatures. The volume specific resistance in C.G.S. units should then be set out in a curve as a function of the temperature. Taking a horizontal line on which to represent decrees centigrade, we set up the values of the volume specific resistance to any scale on the vertical ordinates. For a very small range of temperature this will be found to be nearly a straight line, but for great ranges of temperature the lines (see Dewar and Fleming, Philosophical Magazine, September, 1892, p. 272) will be curved upwards or curved downwards. The line can be produced backwards to the temperature of zero centigrade and the volume specific resistance at that temperature determined. The volume specific resistance of pure copper is an important number. The various deter- minations by Matthiessen and others are not very closely in agreement, but the value generally called Matthiessen's Standard is as follows : The volume specific resistance p of pure soft annealed copper is 1580 C.G.S. units, and the volume specific resistance of pure hard-drawn copper is 1620 C.G.S. It is very usual at the present time to meet with commercial copper wire which gives lower values than the above; that is, which has a higher conductivity than Matthiessen's Standard. The Tables below give the results of measurements of volume specific resistance of various pure metals and alloys by Dewar and Fleming, and also the temperature coefficients (a C.) of the same. (For definition of temperature coefficient see Elementary Form No. 9.) The true temperature coefficient of any material at any temperature is obtained from the temperature resistance curve by taking the slope of the curve at that point, or the trigonometrical tangent of the angle which the geometrical tangent at that point makes with the axis of temperature. For it is obvious that, if R is the resistance JTD at any temperature 0, then -j- is the temperature coefficient at the temperature 0. If a circular-sectioned wire of any metal has a length L centimetres and a mass of AV grammes and a. resistance of R ohms, then it is easily shown that if p 1 is the resistance in ohms of a wire of the same material, having a length of one metre and weighing one gramme, then R= ioooolv' 10000WR or P l = p 1 is obviously the mass specific resistance as above defined, coefficient a measurement of length and resistance suffices. Hence, to determine this Volume Specific Resistances in C.G.S. Units of Pure Metals at 0C., and Mean Temperature Coefficents (a) between and 100C. The Volume Specific Kesistances (p) in C.G.S. Units of certain Alloys at 0C., and Temperature Coefficients (a) at 15C. metals in all cases soit a act annealed Metal. P a Metal. ' a COS Units Platinum- Silver 81582 0-000248 Platinum 10917 003669 Platinum-Iridium 30896 0-000822 Gold 2197 0-00877 Platinum-Rhodium 21142 0-00148 Pal Indium 10219 0-00854 Gold-Silver 6280 0-00124 Silver 1468 0-00400 Manganese- Steel 67148 000127 Copper 1561 0-00428 Nickel-Steel 29452 000201 Aluminium 2665 0-00485 German-Silver 29982 0-000278 Iron* 9065 0-00625 Platinoid 41731 0-00031 Nickel 12823 0-00622 Manganin 46678 0-0000 Tin 18048 0-00440 Silverine 2064 0-00285 Magnesium 4855 0-00881 Aluminium - Silver 4641 0-00288 Zinc 5751 0.00406 Aluminium-Copper 2904 0-00881 Cadmium 10023 0-00419 Copper- Aluminium 8847 0-000897 Lead 20380 0-00411 Copper-Nickel-Aluminium 14912 0000645 Thallium 17688 0-00898 Titanium-Aluminium 8887 0-00290 * The iion here used cannot be considered as pure, but only as approximately pure. Taking a sample of copper or other wire, determine, as above, its volume specific resistance and temperature coefficient at about 15"C. Enter your observations in the form on page 4. DETERMINATION OF THE VOLUME SPECIFIC RESISTANCE AND TEMPERATURE COEFFICIENT OF A SAMPLE OF WIRE. Nature of wire used Length of wire taken Weight of wire in air Weight of wire in water Specific gravity of wire Mean diameter of wire Mean temperature coefficient at centimetres. grammes. grammes. centimetres. C = Observation No. . Temperature of wire. Resistance of wire in ohms, including connectors. Resistance of connectors in ohms. True resistance of wire in ohms. Remarks. TJiese A'otes arc copyright, and all riijl.ts of reproduction are reserved. They are arranged by DE. J. A. FLEMING. of University College, London, and are published by "The Electrician" Printing and Publishing Company, Limited. Salisbury Court. Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. (No. :>). Name Date ZTbe Measurement of Xovv IResistanccs b\> the potentiometer. Tin- i>in-'itiix required for these determinations is a potentiometer set, ivhich 'may be a simple straight wire potentiometer, or a more complete form of the instrument, such as Crompton's potentiometer. The outfit includes a pair of secondary cells of at least thirty to forty ampere-hours capacity, one or more standard Clark cells, and a sensitive suspended coil galvanometer. A set ofloiv resistances unll also be required. The Student is recommended to make a diagrammatic sketch of the arrangement of the A general explanation of the potentiometer and its use has been given in Klementaiy Form No. 10, to which the student is referred. The attention of the student must, however, be directed to the following points in connection with the use of this instrument. THE INSULATION OF THE INSTRUMENT. Difficulties are sometimes experienced in the use of the potentiometer in very accurate work, and when an exceedingly sensitive galvanometer is being used, owing to leakage currents passing through the galvanometer. In some cases merely touching with the hand certain parts of the circuit is sufficient to make galvanometer deflections which altogether mask the real potential differences to be observed. The first point to notice is that the potentiometer, the working secondary cell or cells and the galvanometer should all be insulated by placing them on slips of clean ebonite. The connecting wires should be insulated with gutta peirha or iiuliarubbcr, and in some cases the observer should stand on a sheet of indiarubber or ebonite and have the finger with which he presses the slide key down insulated with a rubber finger-stall. STEADINESS OF WORKING E.M.F. In the next place it is absolutely essential for liood work to have a very steady source of E.M.F. to supply the working current. This can only be obtained by using rather large secondary cells which have been well charged and then partly discharged before using for the potentiometer. The secondary cells give the steadiest E.M.F. when about 25 per cent, of their full quantity has been taken out. If a freshly-charged cell is used it is liable to sudden small changes of electromotive force. GALVANOMETER. The sensitiveness of the potentiometer ultimately depends upon the galvanometer employed with it. If no dynamos or other magnetic bodies are near, a suspended needle galvanometer, such as a high resistance mirror galvanometer, can be used with great advantage ; but if used in a factory this is out of the question, from want of steadiness and on account of disturbing magnetic fields. The dead-beatness and independence of external fields greatly recommend the suspended coil galvanometer. The general conditions with which a sensitive galvanometer for this purpose should comply are, however, that one hundred-millionth of an ampere passing through it should cause a deflection of the spot of light of at least one millimetre when the scale is at a distance of one metre from the mirror. This may be regarded as a good, but by no means an un usual , degree of sensitiveness. The sensitiveness of a galvanometer can be conveniently defined by stating the current through it which will create a deflect inn of one minute of an angle of its coil or needle. The galvanometer circuit should always have, either in the ( 2 ) galvanometer or outside, a resistance of at least 1,000 ohms, in order that the Clark cell may never send a current of more than a very small amount, even in the process of finding the balance. CLARK CELLS. It has been abundantly demonstrated that when Clark cells are made up with pure materials in a certain manner, they preserve for long periods of time a perfect constancy of electromotive force. The following is the Board of Trade specification for setting up Clark cells : ON THE PREPARATION OF THE CLABK CELL. Definition of the Cell. The cell consists of zinc or an amalgam of zinc with mercury and of mercury in a neutral saturated solution of zinc sulphate and mercurous sulphate in water, prepared with mercurous sulphate in excess. Preparation of the Materials. 1. The Mercury. To secure purity it should be first treated with acid in the usual manner, and subsequently distilled in vacuo. 2. The Zinc. Take a portion of a rod of pure redistilled zinc, solder to one end a piece of copper wire, clean the whole with glass paper or a steel burnisher, carefully removing any loose pieces of the zinc. Just before making up the cell, dip the zinc into dilute sulphuric acid, wash with distilled water, and dry with a clean cloth or filter paper. 8. The Mercurous Sulphate Take mercurous sulphate, purchased as pure, mix with it a small quantity of pure mercury, and wash the whole thoroughly with cold distilled water by agitation in a bottle ; drain off the water and repeat the process at least twice. After the last washing, drain off as much of the water as possible. 4. The Zinc Sulphate Solution. Prepare a neutral saturated solution of pure (" pure recrystallised ") zinc sulphate by mixing in a flask distilled water with nearly twice its weight of crystals of pure zinc sulphate, and adding zinc oxide in the proportion of about 2 per cent, by weight of the zinc sulphate crystals to neutralise any free acid. The crystals should be dissolved with the aid of gentle heat, but the temperature to which the solution is raised should not exceed 30C. Mercurous sulphate treated as described in 3 should be added in the proportion of about 12 per cent, by weight of the zinc sulphate crystals to neutralise any free zinc oxide remaining, and the solution filtered, while still warm, into a stock bottle. Crystals should form as it cools. 5. The Mercurous Sulphate and Zinc Sulphate Paste. Mix the washed mercurous sulphate with the zinc sulphate solution, adding sufficient crystals of zinc sulphate from the stock bottle to insure saturation, and a small quantity of pure mercury. Shake these up well together to form a paste of the consistence of cream. Heat the paste, but not above a temperature of 30C. Keep the paste for an hour at this temperature, agitating it from time to time, then allow it to cool ; continue to shake it occasionally while it is cooling. Crystals of zinc sulphate should then be distinctly visible, and should be distributed throughout the mass. If this is not the case, add more crystals from the stock bottle, and repeat the whole process. This method insures the formation of a saturated solution of zinc and mercurous sulphates in water. To set tip the < 'dl. The cell may conveniently be set up in a small test tube of about 2cm. diameter and 4cm. or 5cm. deep. Place the mercury in the bottom of this tube, filling it to a depth of, say, O5cm. Cut a cork about 0-ocm. thick to fit the tube ; at one side of the cork bore a hole through which the zinc rod can pass tightly ; at the other side bore another hole for the glass tube which covers the platinum wire : at the edge of the cork cut a nick through which the air can pass when the cork is pushed into the tube. Wash the cork thoroughly with warm water, and leave it to soak in water for some hours before use. Pass the zinc rod about 1cm. through the cork. Contact is made with the mercury by means of a platinum wire about No. 22 gauge. This is protected from contact with the other materials of the cell by being sealed into a glass tube. The ends of the wire project from the ends of the tube ; one end forms the terminal, the other end and a portion of the glass tube dip into the mercury. Clean the glass tube and platinum wire carefully, then heat the exposed end of the platinum red hot, and insert it in the mercury in the test tube, taking care that the whole of the exposed platinum is covered. Shake up the paste and introduce it without contact with the upper part of the walls of the test tube, filling the tube above the mercury to a depth of rather more than 1cm. Then insert the cork and zinc rod, passing the glass tube through the hole prepared for it. Push the cork gently down until its lower surface is nearly in contact with the liquid. The air will thus be nearly all expelled, and the cell should be left in this condition for at least 24 hours before sealing, which should be done as follows : Melt some marine glue until it is fluid enough to pour by its own weight, and pour it into the test tube above the cork, using sufficient to cover completely the zinc and soldering. The glass tube containing the platinum wire should project some way above the top of the marine glue. The cell may be sealed in a more permanent manner by coating the marine glue, when it is set, with a solution of sodium silicate, and leaving it to harden. The cell thus set up may be mounted in any desirable manner. It is convenient to arrange the mounting so that the cell may be immersed in a water bath up to the level of, say, the upper surface of the cork. Its temperature can then be determined more accurately than is possible when the cell is in air. ( 3 ) In using the cell sudden variations of temperature should, as far as possible, be avoided. The form of the vessel containing the cell may be varied. In the II form, the zinc is replaced by an amalgam of 10 parts by weight of zinc to 90 of mercury. The other materials should be prepared as already described. Contact is made with the amalgam in one leg of the cell, and with the mercury in the other, by means. of platinum wires sealed through the glass. Several cells should be set up according to this specification and tested against est done by pressing the flattened and cleaned ends of the potential wires underneath the brushes and thus causing them to press against the commutator segments or collector rings. These two pairs of potential wires arc then brought to four mercury cups placed near the galvanometer. The galvanometer should be set up in a steady place and have a uniformly divided scale. The image reflected on to the scale should be the image of a fine wire illuminated from behind, so as to get scale readings of the deflection of the coil as. sharply as possible. AVhen all is ready the plug key is closed, so as to send the current through the armature and standard resistance. The potential wires from the ends of the armature and resistance are to be connected alternately to the galvanometer. It will usually be necessary to add a high resistance in series with the galvanometer to reduce the deflection to a convenient amount, but it should be as large as possible, and it should be possible to read this deflection to at least one per cent, of its value. Let the deflection be noted and called d 2 , when the potential wires from the ends of the known low resistance are connected to the galvanometer. In the same way let the deflection be noted and called d^ when the potential wires from the ends of the arma- ture are connected to the galvanometer. It is well to take the deflection due to the fall of potential down the known low resistance before and after that due to the armature, and then to take the mean of these two results. In this way we eliminate any error due to the sinking of the electromotive force of the battery during the experiment. These deflections d and d z are proportional, or nearly so, to the currents flowing throuo-h the galvanometer in the two cases. When the galvanometer is connected to the ends of the standard resistance, or to those of the armature, a current flows through the galvanometer and associated high resistance, which is numerically equal to the quotient of this difference of potential. by the total resistance of the galvanometer current. Let Ej be the unknown resistance of the armature in ohms, and let V x be the fall in potential in volts down it when the steady current is flowing through it. In the same way, let R 2 be the resistance of the known low resistance standard, and let V 2 be the fall in potential down it. Then the current flowing through the armature is equal v V to Xi amperes, and the current flowing through the standard resistance is S. But these Rj **st currents must have the same value because the standard resistance and armature are in series. Hence, v t ^ 2 Tv.B, B,~- R; v 2 ~ K.' Acrain, if r be the resistance of the galvanometer and its associated high resist- ance : then, when the potential wires from the ends of the known low resistance are put in connection with the galvanometer, the current through the galvanometer is equal ( 3 ) V r meter constant, we have to 2 amperes, and if the deflection of the galvanometer is d t , and if C is the galvano- In the same way, if rk; and we can always, if need be, calibrate the galvanometer scale so a> i" ascertain what the deflections really mean. In one of these two ways the student should practise measuring the resistance of several armatures, and also of the low tension coil of several transformers, recording this result in the appended form. THE MEASUREMENT OF ARMATURE RESISTANCES. B 2 = Standard low resistance used = ohm. Observation No. Galvanometer deflection or potential difference at ends of standard Galvanometer deflection or potential difference at ends of Calculated value of the unknown resistance R,. Ra=RA Description of armature or coil resistance. Copper Deposit The apparatus required for these experiments is a good chemical balance, which must be <</( of \veighing a mass of 100 grammes with an accuracy ofO'l of a milligramme, a. large copper depositing cell as described Mow, a source of constant current, some regulating resistances, and instruments to calibrate. Studi-nt is recommended to sketch the arrangement of the circuits and apparatus. The passage of a current of electricity through a solution of copper sulphate by means of copper electrodes causes copper to be dissolved off one plate or electrode and deposited on the other. The first plate is spoken of as the loss plate and the second as the gain j>tate. If certain precautions be taken, as stated below, the amount of copper deposited on the gain plate in a second bears a definite and fixed relation to the average strength of the current passing through the electrolyte. In exact researches there are certain advantages to be gained by the use of a solution of silver nitrate, and silver plates instead of copper sulphate and copper plates, and the British Board of Trade have adopted the silver deposition as a basis for the practical determina- tion of current strength. The legal definition of the ampere is as follows : One ampere is the denomina- tion of a current of unvarying strength which deposits silver at the rate of O'OOlllS of a gramme per second. The weight of metal deposited by one ampere per second is called the electro- chemical equivalent of the metal. The following are the directions given by the Board of Trade for determining the strength of a current by the method of silver deposit : In the following specification the term silver voltameter means the arrangement of apparatus by means of which an electric current is passed through a solution of nitrate of silver in water. The silver voltameter measures the total electrical quantity which has passed during the time of the experiment; and by noting this time, the time average of the current, or, if the current has been kept constant, the current itself can be deduced. In employing the silver voltameter to measure currents of about one ampere, the following arrange- ments should be adopted. The cathode on which the silver is to be deposited should take the form of a platinum bowl not less than 10cm. in diameter, and from 4cm. to 5cm. in depth. The anode should be a plate of pure silver some 80 sq. cm. in area and 2mm. or 3mm. in thickness. This is supported horizontally in the liquid near the top of the solution by a platinum wire passed through holes in the plate at opposite corners. To prevent the disintegrated silver which is formed on the anode from falling on to the cathode, the anode should be wrapped round with pure filter paper, secured at the back with sealing wax. The liquid should consist of a neutral solution of pure silver nitrate, containing about 15 parts by weight of the nitrate to 85 parts of water. The resistance of the voltameter changes somewhat as the current passes. To prevent these changes having too great an effect on the current, some resistance besides that of the voltameter should be inserted in the circuit. The total metallic resistance of the circuit should not be less than 10 ohms. Mithod nf making a Measurement. The platinum bowl is washed with nitric acid and distilled water, dried by heat and then left to cool in a desiccator. When thoroughly dry it is weighed carefully. It is nearly filled with the solution, and connected to the rest of the circuit by being placed on a clean copper support to which a binding screw is attached. This copper support must be insulated. Of ( 2 ) The anode is then immersed in the solution so as to be well covered by it and supported in that position ; the connections to the rest of the circuit are made. Contact is made at the key, noting the time of contact. The current is allowed to pass for not less than ha)f-an-hour, and the time at which contact is broken is observed. Care must be taken that the clock used is keeping correct time during this interval. The solution is now removed from the bowl, and the deposit is washed with distilled water and left to soak for at least six hours. It is then rinsed successively with distilled water and absolute alcohol, and dried in a hot-air bath at a temperature of about 160C. After cooling in a desiccator it is weighed again. The gain in weight gives the silver deposited. To find the current in amperes, this weight, expressed in grammes, must be divided by the number of seconds during which the current has been passed, and by O'OOlllH. The result will be the lime-average of the current, if during the interval the current has varied. In determining by this method the constant of an instrument the current should be kept as nearly constant as possible, and the readings of the instrument observed at frequent intervals of time. These observations give a curve from which the reading corresponding to the mean current (time-average of the current) can be found. The current, as calculated by the voltameter, corresponds to this reading. For most purposes the deposit of copper can be made to give results of almost equal exactness with silver, provided that certain precautions are taken. To determine the strength of a steady or unvarying current by the method of copper deposit, and to check an ammeter reading at the same time, the following arrangements are made : A glass cylindrical vessel is taken and filled nearly to the top with a slightly acid, solution of sulphate of copper. In this solution are suspended an odd number of copper plates, alternate plates being connected together. This vessel is called a copper voltameter. The size of plates, and therefore of the voltameter cell, is determined by the strength of the current it is desired to measure, by the rules given below. The 1st, 3rd, 5th, 7th, &c., plates are metallically connected, and these plates are made the loss plates. The 2nd, 4th, 6th, &c., plates are also metallically connected, and are made the gain plates. If there are N gain plates and each has a total surface area of S square centimetres, reckoning both sides, then N S square centimetres is the total area of gain plates ; and, similarly, if S is the total surface of the single loss plate, and there are N + 1 loss plates, the total opposed surface of the loss plates is also N S square centimetres. The total surface of loss plates should never be less than 40 square centimetres per ampere. If the area is much less than this amount the resistance of the cell becomes high and variable, owing to the formation of copper oxide on the plate. The gain plates should never expose a less area than 20 square centimetres per ampere, or else the copper deposit is not firmly adherent to the plate, and a portion of it may be lost in washing or drying. It is always safest to employ from 50 to 100 square centimetres of plate surface per ampere. The size of electrolytic cell or voltameter is therefore determined by the currents to be measured. Assuming that a current, say, of about five amperes is to be measured, proceed as follows : The current must be supplied from secondary cells, and be regulated by a rheostat or resistance capable of being varied by exceedingly small steps. One of the most convenient forms is a carbon plate rheostat, in which the resistance is varied within certain limits by squeezing more or less tighlly a series of carbon plates, about 3 inches square, held in a frame. The current passes through the ammeter or ampere-balance to be checked, and then through a voltameter with copper plates of such size and number that we have about 50 square centimetres of surface per ampere. The copper plates must be so held in this cell that they cannot move during the experiment, but be capable of being easily taken out of the cell for the purpose of weighing. Details of the best way of doing this will be found in a Paper by Mr. A. "NY. Meikle, read before the Physical Society of Glasgow University, January 27th, 1888, or in one by Mr. Thomas Gray, published in the Philosophical Magazine for November, 1886. The copper sulphate solution must be made by dissolving pure crystals of the salt in distilled water until a solution of a density of I'lo to 1'18 is obtained. One per cent, by volume of free strong sulphuric acid must then be added, and this addition of free acid is essential to success. No good results can be obtained with neutral solutions. The copper plates must be cleaned by dipping them in strong nitric acid and washing and drying carefully. They ought to be perfectly bright and clean, and not afterwards touched with the fingers. Each plate should have a number stamped on it for rcc( ignition. The edges of the plates and sharp angles should be first rounded off with the file. Each plate must then be carefully weighed. Before weighing, the plates must be kept for some time in a desiccating chamber over strong sulphuric acid to remove the film of moisture adhering to them. The plates, when weighed, are then to be arranged in the voltameter and connected up in the proper manner. The voltameter is connected in series with the ammeter to be checked, and at a known instant of time the current is started through the cell. The observer has then to keep the current as constant as possible by means of the variable carbon resistance, mid this constant current must be kept flowing through the cell for some hours. The duration of the experiment must be observed by a good clock or watch, as the absolute determination of the time is involved. At the same time the scale reading of the ammeter to be checked is carefully taken. When a sufficient time has elapsed the current is stopped and the plates removed from the electrolytic cell The gain plates should be taken out at once and be given a rinse in water to free them from adhering solution. A few drops of sulphuric acid must be added to this water to prevent oxidation. The plates are then dried on clean white blotting paper, and put into a copper hot air oven for a few minutes to dry completely. They are then transferred to the desiccator to cool and finally weighed. The exact gain of each plate is noted, and the total gain obtained by addition. The gain in weight per second is then calculated and divided by the electrochemical equivalent of copper, and the resulting quotient is the value of the mean or average current in amperes. The electrochemical equivalent of copper varies very slightly with current density and temperature as follows : Area of gain plates in square centimetres Electrochemical equivalent of copper per ampere. at 12"C. at 28C. 50 0-0008287 0-0003286 100 0-00032H4 0-0008283 150 0-0008281 0-0003280 200 0-0003279 0-0008277 250 0-0008278 0-0008275 800 0-0003278 0-0003272 The Student should make in this way a careful check of the reading of an ammeter or current-balance at one or two points on the scale. The results should be entered up in the form on the next page. STANDARDIZATION OF AN AMMETER BY COPPER DEPOSIT. Observation No Number of plate. Weight of (gain) copper plates Weight of (gain) copper plates at Total gain in weight of Duration of experiment. Calculated mean value of the Reading of ammeter or at beginning. end. plates. current. galvanometer. These Notts are copyright, and all rights of reproduction are reserved. They are arranged by DK. J. A. FLEMING, of University College, London, and are published by " The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. 25. APVAWCEP (No. 5). Name Date TTbc Stanfcarbisation of a Doltmeter b\> the potentiometer. The apparatus needed for these tests is a complete potentiometer si't, preferably Crompton's form of the instrument, and a divided resistance consisting of , a coil or coils of wire, divided in known and fixed ratios. The galvanometer employed icith the potentiofneter should be a very sensitive movable coil galvanometer, and the Clark cells used should fiare been checked against reliable standard cells for comparison. I ' i Tlie, Student should sketch the arrangement of apparatus as set up. The general arrangement of the simple wire potentiometer has been fully described in Elementary Form No; 10. For most exact wprk it is better to employ the more complete form of instrument arranged by Crompton. In this instrument only a part of the resistance wire is actually a stretched wire ; the major part consists of coils of wire wound on bobbins placed underneath the base of the instrument. A more detailed description of this potentiometer may be given with the help of Fig. 1 (see page 2). , : A B is a short stretched manganin wire, and one epd of it is connected with 14 exactly similar coils of wire, joined in series, each of which measures about twa ohms. These 14 coils, marked 1 to 14 E, are arranged so that the contiguous ends of each coil are brought to a set of 14 studs arranged in a semicircle on the board. The ends of this resistance, consisting of the 14 coils of wire and the straight stretched wire equal in resistance to one of them, constitute the potentiometer wire, and it has a total resistance of about 30 ohms. The end of this series is connected to another set of 14 coils of wire, G, joined in series, each equal to two ohms in resistance and arranged with contiguous ends joined to 14 studs on the board, and the last coil is connected to a spiral sliding resistance or rheostat G l9 so that a graduation of resistance may be made. To the ends of this complete resistance is joined one cell of a secondary battery, which constitutes the working cell of the potentiometer. The connections are made as in the Diagram in Fig. 1 in (No. 6) Advanced Form. By turning the switch handle G to rest on different studs of the set of 14 coils, and varying the sliding resistance G, by turning the disc, we can introduce resistances varying from zero to about 30 ohms in series with the other 14 coils E and stretched wire A B. By this means we can always make the potential difference between the ends of the resistance consisting of the 14 coils and straight wire exactly equal to 1'5 volts. The straight manganin wire lies on a scale divided into 1,000 parts. Hence, when the fall of potential down the wire and coils is l - 5 volts, the fall down a length of the wire equal to one of the scale divisions is one ten-thousandth of a volt. Along and over the wire moves a slider, C, which can make a contact with the wire at any point. One terminal of a galvanometer is attached to the centre of the radial contact arm E, which works over the studs of the 14 coils so as to make a contact at any stud', and the other end is joined to one terminal of a double- pole radial switch H, the second arm of which is joined to the sliding contact piece on the man- gauin wire. This radial switch can be moved over to make double contacts with a set of four or six pairs of studs, and these studs are in connection with a set of four or six pairs of terminals on the front of the base board marked 1 to 4, by means of which any cell or source of electromotive force may be inserted in series with the galvanometer and slider. We have then also .to provide a pair of standard Clark cells, made as described in (No. 2) Advanced Form, and a very sensitive mov- able coil galvanometer. For volt- meter checking we have in addition to provide a divided resistance or volt box. This consists of a coil of mangaiiin wire of about 5,000 or 10,000 ohms in resistance, and which is divided in certain exact ratios, so that one section is, say, 50 ohms, and another 100, and another 500 ; so that we have a resistance which will bear a terminal potential difference of 100 to 150 volts put upon it. We are then able to make contacts through the terminals with points on this re- sistance between which there is Ttjo tn > Tftr tn > or xV tn f tne po- tential difference there is between the ends of the whole resistance. To check and calibrate a voltmeter with this apparatus we proceed as follows: Suppose the voltmeter reads from 60 to 100 or 120 volts. Provide a set of 50 small ( 3 ) secondary cells. For this purpose none are more convenient than the small lithanode glass tube secondary cells, but they must be arranged so that any number of cells can be employed. Join up the volt box or divided wire across the terminals of the voltmeter to be tested, and connect, say, 50 cells across the terminals. The voltmeter will now read nearly 100 on its scale. To find out what the potential difference really is, bring wires from the { ,' )0 th section of the volt box resistance to the potentiometer, and connect them in, as described in Elementary Form No. 10, to one of the pairs of terminals; join in also a couple of Clark cells to any other two pairs of terminals on the potentiometer. See that the potentiometer and galvanometer are all carefully insulated, and begin by setting the potentiometer by the Clark cell by varying the resistance of the series of twelve coils and spiral resistance, so that the fall of potential down the 14 coils and slide wire is exactly T434 volts for 14340 scale reading; that is, if the E.M.F. of the Clark cell is 1'434 volts at that time, then the Clark cell must be made to balance at 14340 on the scale. This is achieved by setting the radial arm E of the 14 coils to touch stud 14 and setting the slider on the manganin wire to touch at 340 on the scale of the slide wire, and then varying the resistance of the other coils G and spiral resistance Gj until the galvanometer shows no deflection. When this is the case the fall of potential down the slide wire is '0001 volt per division. The two Clark cells should then be compared, and, if in agreement, the double- pole working arm H should be switched over so as to throw in one hundreth part of the potential difference on the terminals of the voltmeter. The switch arm of the 14 coils will now have to be moved over to another stud on series E, say to stud 9 or stud 10, and a position of galvanometer balance found by moving the slider. Suppose the balance is found with arm on stud 10 and slider at 125. This indicates that the potential difference at terminals of voltmeter is 101*25. If the voltmeter reads 100 on its seiile, the voltmeter error is +1*25 volts at 100. In this way, by altering the number of battery cells, the voltmeter may be checked all along its scale. The student should make an accurate check in this way of one or more voltmeters, and set out a <;urve of errors by drawing vertical lines above or below a horizontal datum line (which lines are equal on some scale to the + or errors), and setting these lines at points on the equally divided horizontal line chosen to represent the scale divisions of the voltmeter. The results should be entered up in the form on page 4. STANDARDIZATION OF A VOLTMETER. Observation No Temperature. Value of Clark cell for that Scale reading on potentiometer for fraction Divided resistance used. True value of potential difference Observed voltmeter reading. Error of voltmeter. tempera- ture. of voltmeter potential. of voltmeter terminals. ' These Notes are copyright, and all rights of reproduction are reserved. They are arranged by DR. ,T. A. FLEMING, of University College, London, and are published by "The Electrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. 26 ADVANCED (No. 0). Name. Date Stanbarbisatton of an Hmmeter b\> the potentiometer. Tlie apparatus required for these experiments is a complete potentiometer set (Crompton'a form) as described in (No. 5) Advanced Form and, in addition, a set of low resistance standards prepared or copied from knoivn standards as in (No. 2) Advanced Form. Arrangements must be made for taking the required current from large secondaiy cells and regulating it by resistance KX required. Tin' Student in recommended to sketch the arrangement of apparatus as set up. To standardize an ammeter by the potentiometer, this last instrument is set up and adjusted in accordance with the instructions given in Advanced Forms (No. 2) and (No. 5), which the student is assumed to have read. A diagram of the connections is shown in Fig. 1. ll RneostRts. .AAAAAAA. -A* ' * ' 'MAAAAAAAAA'A Ir- Secondary. 90 80 70 60 50 WWVW - VVkrAiiVsVVVVVVVYVV*^ 40 C 30 20 1 2 4 6 a 10 12 II 3 SCALE 1100 volts. "*" Galvanometer. L^>=-^_ Measurement of *t Hicrh Vnlinpp<; 7 ?! A x 1 tti High Resistance. 199 C59?!>!'qc30itD05Qcoj>ooooooooi(oooiiooteoco T oooi U Standard 1 Current Measurement. Cell. Comparison of low voltages. ' 1 ft Standard Resistance.!*) ; ' 1 f\ iAiA>i. T7" P ( Amme Calibration.! g 1 ,^; Ammeter, Adjustable or Resistance. dard ReRist.nr.a. FIG. 1. The. ammeter to be calibrated is set up and joined in series with a known low resistance .standard, which will carry currents of the strength required without sensible heating. Thus, if the ammeter reads from to 10 amperes, we should select a O'l ohm standard, which will carry 10 amperes without sensible heating. These low resistance .standards are best made of gilt manganin strip. The ammeter and resistance are joined in series with a carbon rheostat, which consists of plates of carbon about 3 or 4 inches square, and which can be more or less compressed with a screw. These three pieces of apparatus ammeter, carbon resistance and standard resistance should be connected to one or more large cells of a secondary battery sufficient to provide a steady current of ( 2 ) the maximum magnitude necessary. This being done, the ends of the low resistance standard are joined by potential wires to the potentiometer, care being taken to send the current through the standard in the right direction for fall of potential (.srr Elementary Form No. 11.). A Clark cell is connected to the potentiometer and the potentiometer set by it. We then measure the fall of potential down the low resistance standard, and read at the same time the ammeter indication. The current through the ammeter is then changed to another value and the same measurement repeated. Suppose the fall of potential down the O'l ohm standard is found to be '951 volt, and the reading of the ammeter at that instant is 10"2, this indicates that the true current through it is 9*51, and the scale error is therefore + '69. The student should in this way check a few ammeters and record the scale errors at various points of the scale. In thus checking an ammeter it is always necessary to reverse the current and check it at all points of the scale, both with increasing currents and with decreasing currents, because some types of ammeter in which soft iron is employed are quite dif- ferent in their reading under these conditions. A good ammeter should comply with the following requirements : (i.) It should be as dead beat as possible; that is, the needle should come very quickly to rest when the current is sent through it. (ii.) It should give the same scale reading for the same current whether that scale reading is reached by increasing from a smaller current or de- creasing from a larger one. (Hi.) It should not be affected by external magnetic fields. (iv.) It should be sensitive ; that is, a very small change in the current should show itself immediately on the scale reading of the instrument. (v.) Other desirable requirements are that the scale divisions should be equal in magnitude throughout the scale, and that it should begin to read from the zero point, and not have a blank space of non-useful scale. The above requirements should also be fulfilled by a good voltmeter, and in addition this last should have a negligible or known coefficient of temperature correction. These conditions are not always fulfilled by instruments in which soft iron cores are employed which are moved in magnetic fields. Hence the above tests should always be applied in examining any instrument before passing an opinion upon its merits as an ammeter. The student will find it a useful exercise to take some good form of ammeter or current balance and check it at some point on the scale, both by the copper deposit method, as described in Advanced Form (No. 4), and also to check it b}' the potentiometer method as above described, and see how far these entirely different physical methods lead to the same result. By the use of the potentiometer all measurements made in the electrical labora- tory with continuous currents may be ultimately reduced or referred to a standard of electromotive force represented by a Clark cell and a standard resistance. The student should enter up his observations in checking any ammeters in the appended form. STANDARDIZATION OE AN AMMETER. Observation No Temperature. Value of E.M.F. of Clark cell at that Potentiometer reading or potential difference at ends of Value of standard resistance True value of current in Scale reading of Error of ammeter. temperature. standard resistance. employed. amperes. ammeter. STANDARDIZATION OF AN AMMETER. Potentiometer Observation No Temperature. Value of E.M.F. of Clark cell reading or potential difference Value of standard resistance True value of current Scale reading of Error of ammeter. temperature. standard employed. amperes. ammeter. resistance. . - These Notes are copyright, and all rights of reproduction are reserved. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The Electrician" Printing and Publishing Company Limited Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. No. 27. APYAJJCEP (No. 7). Date Determination of the flfeaonetic permeability of a Sample of Jvon. The apparatus required for these experiments is a set of soft iron rings, circular in section, and of which the diameter of cross-section is not large compared icith the mean diameter of the ring. A ballistic galvanometer and con- i /i- user fur standardizing it is required, and also n large soft iron ring cut into tivo parts. The Student is recommended to sketch the arrangement of the apparatus as set up for this test. If a very long rod of iron is placed in a long magnetising coil, or solenoid, and is magnetised by a uni-directional current sent through the coil, the magnetic force in the interior of that solenoid can be calculated, as shown in Elementary Form No. 7. If the iron rod is, however, a short one that is to say, if it is less than about 50 diameters long the magnetic poles induced in the ends of the bar make their effect felt in diminishing the magnetic force due to the coil in the space occupied by the rod. Assuming the rod, however, to be a long one that is to say, 100 or more diameters long we can calculate the magnetic force H, due to a current of A amperes circulating in the solenoid of N turns and length L, by the formula H = 4 1 AN 10 L If a secondary circuit of wire is wound closely round the centre of the irou rod, and if the current in the magnetising solenoid is reversed suddenly the secondary circuit being connected with a ballistic galvanometer it is shown in Elementary Forms Nos. 7 and 8 that we can calculate the induction density B in the iron at the place where the secondary coil is wound. The magnetic induction density B, or, as it is commonly called, the number of lines of force per square centimetre, and the magnetic force H, are related to one another by the equation B = /tH, where /* is the magnetic permeability of the material. The object of the following experiments is to determine the magnetic permeability of a sample of iron. Whenever that iron can be furnished in the form of a ring, the determination is very simple. The iron ring should be turned up in the lathe and formed so that it has a circular cross- section of radius a and a mean diameter of 2r. These measurements should be taken in six different places and the mean value obtained. The ring may be, preferably, about one centimetre in diameter of cross-section and ten centimetres in mean diameter. The ring should then be carefully wound over with N turns of insulated wire in one layer No. 18 S.W.G., double cotton or silk-covered copper wire being used, and the mean diameter of the cross-section again taken over the wire. Let this be 2b centimetres. Then - - is the mean radius of each circular turn of wire put on the ring. Let there be N turns of wire in all on the ring. Then the mean magnetic force H inside the wire windings can be calculated, and is equal to 4 NA ( /- - 2 ) To-srrV^-"}- . i For proof of this formula see Fleming's " Alternate-Current Transformer," Vol. I. Appendix, Note B. Second Edition. To determine the induction which this force produces, wind upon the ring over the primary coil a small secondary coil consisting of ten or twenty turns of No. 36 S.W.G. silk-covered copper wire, interposing a layer of silk between the primary and secondary coils. This secondary coil must be connected with a ballistic galvanometer through a variable resistance. The primary coil must be connected with a battery through a reversing switch which will enable the primary current to be iustantaneously reversed. These preparations being made, begin by standardizing the ballistic galvano- meter with a standard condenser in the manner described in Elementary Form No. 5, and obtain the ballistic constant, or the number by which the sine of half the ano-le of throw r of the coil or needle, when corrected by the logarithmic decrement factor, must be multiplied to obtain the quantity of electricity in micro-coulombs which passed through the galvanometer to produce that deflection. Then place an ammeter, carefully standardized (preferably one of Weston's ammeters), in series with the primary coil, and begin a series of observations. Close first the primary circuit, and observe the value of the primary current A on the ammeter in amperes. Then bring the ballistic galvano- meter to rest, and connect it to the secondary coil. Suddenly reverse the primary current, and note the throw 20 of the image of the spot of light reflected from the mirror of the ballistic galvanometer. One or two preliminary trials may be necessary in order to determine exactly the best resistance to have in the galvanometer circuit to obtain the greatest value of the throw of the coil or needle. Measure the total resistance of the secondary circuit, consisting of the galvanometer, secondary coil resistances, and leads. Let this altogether be R ohms. Then, if C is the ballistic constant of the fi S ~\ "\ galvanometer, we know that C sin -- ( 1 + - Y where X is the logarithmic decrement of the galvanometer, is the value in micro-coulombs of the quantity of electricity sent through the galvanometer by reversing A amperes in the primary coil. If the value of the induction density, or number of lines of force per square centimetre, in the iron core is called B, if S is the cross-section of the iron core, and if n is the number of turns of curve on the secondary coil, then B S n is the total induction through the secondary coil, and, as in Elementary Form No. 8, B Sw = - R C sin * (l + -) 10 9 x KT 7 . Hence we can calculate the value of B for R C sin f (l + $) 10 9 "D 2 Sn 1G 7 where the factor 10 9 comes in to reduce the resistance R measured in ohms to absolute ( 3 ) C.G.S. units and the factor 10~ T to reduce the quantity measured in micro-coulombs to- absolute C.G.S. measure. B is given then in C.G.S. measure. The factor 2 or -- comes a in in the above equation because the current of A amperes is reversed in the experiment and not merely .stopped. Hence we have the means of calculating the value of the magnetic force H in the place where the iron is, and also of measuring the value of the induction density B in it ; and hence we can obtain the value of the permeability /i by taking the ratio of B to H. If the sample of iron cannot be procured in the form of a ring we may proceed as follows : Prepare as above two half rings of best soft iron of highest permeability attainable, and measure and wind them as above. Let the end faces of the half rings be perfectly plane, so as to fit together truly. Prepare the iron of which the permea- bility is to be taken in the form of two small cylinders about one centimetre in diameter and one centimetre in length, and let the end faces be truly plane and parallel. Wind secondary coils as above over these little cones. Arrange these small test pieces with coils between two half circular rings of soft iron of one centimetre in diameter in cross section, and press the whole together firmly. Then proceed to take the permeability as above described, using both the secondary coils as search coils in separate experiments. The primary coils on the two half rings must, of course, be joined in series. In calculating the magnetic force H, to which the test pieces are sub- jrrtrd, we may consider that it is nearly the same as would exist if the ring built up of the two half iron rings and two test pieces formed one single undivided ring. Hence, if I is the length of the test piece, and if /) is the mean length of each half iron ring, the whole mean length of the magnetic current, when put together, is 2 (l + li), and the mean radius of this compound ring must be considered to be equal to i\ where 2jr ; - i = 2 (l + Ii), and this value of i\ must be used instead of > in the above formula forH. In order to secure good results the test pieces must have their end surfaces exceedingly true and flat, and must fit exactly against the true surfaces of the ends of the half ring magnets. The induction then flows uniformly through the test pieces without leakage, and the secondary coil wound on the test piece gives the value of this induc- tion. The student should in this manner make measurements of the permeability of samples of iron and steel, and for fuller information on this subject he may refer to the following works : "Magnetic Induction in Iron and other Metals," Prof. J. A. Ewing, Chap. III., p. 59. Article, " Magnetism," in " Encyclopaedia Britannica," IXth Edition, Vol. XV.,. p. 256. Mascart and Joubert, " Electricity and Magnetism," Vol. II., p. 637. The Student should enter up his results in the form on the next page. DETERMINATION OF MAGNETIC PERMEABILITY. Mean diameter of iron ring = Mean diameter of cross-section of iron ring = Number of turns of primary coil Length of test piece = Diameter of test piece = Number of turns of secondary coil Resistance of whole galvanometer circuit = Observation No... Current in primary coil in amperes. Angular deflection of galvanometer needle on reversing primary current = 6. Value of sin - 2 Calculated value of magnetising force = H. Calculated value of magnetic induction, B. Value of Permeability B These Notes are copyright, and all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England, ELECTRICAL LABORATORY NOTES AND FORMS. JXo. 28. ADVANCED (No. 8). ,\'tit>(> Date Stanbarbisatfon of a Ibtob tension Doltmeter. /// tl,,'s,' tests it is assumed flint the Laboratory is supplied with alternating currents,' ii-li ifh can be raised to a high pressure by transformers. The most con- n- a ifnt njiji/i-tnce for laboratory ivork is an alternator coupled direct to a continuous current motor, the motor being driven by current from secondary batteries. By suitable resistances it is then possible to drive at constant speed and to generate a steady alternating current at a pressure of, say, 100 rolls. This can be raised by a small transformer to 1,000 to 3,000 volts. A series of divided resistances are required and also a potentiometer set. Tin' Student should carefully sketch out a diagram of the arrangement of apparatus required before proceeding to work. NOTE. In all tests with high voltage currents great care should be taken to avoid accidents by shock. India-rubber gloves should be used by the operators, r surfaces and a set of plates or surfaces which are movable round an axis or wire suspension. When a difference of potential is made between these fixed and movable plates there is an attractive force exerted between them which moves the movable plates against a restoring force due to gravity or to torsion. High tension electrostatic voltmeters of this class are very useful for alternating current measurements, because they arc unaffected by the frequency of the alternations. In order to calibrate such a voltmeter, it should be fixed in position, well insulated, and then connected with the source of alternating electromotive force. The above-mentioned combination of motor and alternator is especially useful for such calibration work, because the value of the alternating potential is so easily changed and controlled by resistance inserted in the field of the continuous current motor. Suppose the voltmeter reads from 1,000 to 2,400 volts. \ve have then to provide a non-inductive resistance capable of ( 2 ) being put in series across the high tension circuit and kept there for any length of time. The most convenient and safe resistances for this purpose are Dr. Fleming's non- inductive resistance cages of platinoid wire, which are made up in cages of 100 ohms or more. Twenty of these joined in series make a useful form of high tension resis- tance. From the ends of a section of one-twentieth of this resistance potential wires are led down to a low volt electrostatic voltmeter, preferably Lord Kelvin's horizontal pattern. The arrangement then is as follows: The high tension voltmeter and high resistance are placed across the high tension mains, and a pair of potential wires are then joined to a small section of this resistance and brought to a low tension voltmeter. The low tension voltmeter must first have been calibrated carefully by the potentiometer. In many forms of electrostatic voltmeter there is a small difference in the reading on a continuous current circuit, according to whether the needle of the electrostatic volt- meter is made positive or negative, and the difference between these readings may amount to a third or half a volt. Hence the low tension voltmeter must be calibrated with the potentiometer, both with the needle positive and the needle negative, and the mean value of these corrections taken as the correction for alternating currents. Thus, if the voltmeter reads 99'5 when 100 volts is applied to it by the potentiometer, needle being negative, and 99 - 8 when the needle is positive under the same conditions, we take 99'65 as the reading when 100 alternating volts are applied to it, and the scale error at this point is 0'35 for alternating pressure. The low pressure voltmeter being calibrated, we have then to measure very carefully the ratio in which the resistance is divided. This is done by measuring the whole resistance, and the resistance of the two sections. Let the whole resistance be R ohms, and the resistance of the small section v r ohms, then -^ is the fractional division of the wire. The alternating pressure is then varied so as to put different pressures on the terminals of the high tension volt- meter. The corresponding reading of the low tension voltmeter is taken. If the high tension voltmeter reads a value, say, V volts by its scale, and if the corrected value of the low tension voltmeter at that instant is v volts, then the true value of the high TD tension pressure is - *- v volts, and the error of the high tension voltmeter at that point on the scale is VK. - v. r There is one curious possible cause of error. The electrostatic voltmeters, although generally supposed to take no current, really have a small but definite capa- city, and take a definite but small alternating current through them. Hence, if the value in ohms of the divided resistance is very large, say 40,000 or 50,000 ohms, the X 3 ) actual current flowing through this resistance may not be so very different from the capacity current of the electrostatic voltmeter used for the low readings, and this last may shunt a sensible fraction of the current. In other words, it can be shown that the ratio of the fall of volts down the whole, and down the small section of the resistance, is then no longer in the exact ratio of the whole to the small .section of the resistance. Hence it is essential to have a resistance which does not take too small a current. In testing high tension voltmeters up to 2,000 volts, it is best to have a resistance of about 4,000 ohms, which will carry a current of 0'5 ampere, and to divide this resis- tance in the ratio of 20: 1. The smaller section then has a resistance of 200 ohms, and the half ampere carried by it is very large compared with the capacity current of a low reading electrostatic voltmeter. Of course this source of error only comes in with alternating currents. Since the correction to be applied in the case when we are using a very high resistance involves a complicated function of the resistances and inductances of the sections of the wire as well as a knowledge of the capacity of the voltmeter, and since these last two quantities are difficult to measure accurately, the best plan is to avoid the necessity for correction altogether by employing a non- inductive divided resistance, taking a current of not less than 0'5 ampere when placed across the high tension circuit ; and this, together with the use of one of Lord Kelvin's multicellular electrostatic voltmeters to read the low tension volts, will eliminate any necessity for correction in most cases. The Student should in this manner calibrate and check one or two commercial high tension voltmeters, and refer all the readings to the E.M.F. of a Clark cell. The results should be entered up in the appended form. NOTE. H. T. is a common abbreviation for the words high tension, and similarly L. T. for the words lou~ tension, these terms being applied broadly to pressures of about 100 volts and under and to pressures of 500 volts and upwards. STANDARDIZATION OF A HIGH TENSION VOLTMETER. Observation No. Scale reading of H.T. voltmeter Value in ohms of whole resistance Value in ohms of small section of Corrected scale reading of L.T. Error of H.T. voltmeter R Description of H.T. voltmeter =v. used = K. resistance r. voltmeter = V - v. r used. t , - . Thete Notes are copyright, and all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They ;>re arranged by DR. .1. A. FLEMING, of University College, London, and are published by "The Electrician " Printing anil Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. 29. ADVANCED (No. 9). Name Date Examination of an Hlternate^Gurrent Hmmeter. The apparatus required for these tests is a carefully standardized ammeter ivhich has been calibrated by continuous currents with the potentiometer in the manner described in the Advanced Form (No. G). One of the Weston instruments or Kelvin ampere balances is a good one for this purpose. The alternating cuii'ent ammeter to be checked is put in series ivith this standard, and is calibrated by continuous currents by reference to the standard direct- current ammeter. The Student is recommended to sketch carefully the arrangement of the apparatus as xef up. An alternating current is one which reverses its direction at regular intervals, and passes through a uniform cycle of values. The number of complete cycles per second is called the frequency of the current. If a horizontal line is drawn, and divided into equal parts to represent equal small intervals of time say, one-thousandth of a second and if at each of these points an ordinate is erected representing in direction and magnitude the current in the conductor at that instant, the extremities of these ordinates delineate a curve which is called the current curve, and which has very different forms in different cases. In some cases it is nearly a simple sine curve ; in others, quite different. If the time line is divided into very numerous equal parts, and if ordinates are drawn ;it each of these divisions to meet the curve, the square root nftlie average value, of the square of the length of each ordinate is called, for shortness, the mean-square value of the ordinate. This mean-square value (which is written v/meair) is an important quantity. (For further information the Student should consult "The Alternate-Current Transformer," by Fleming, Vol. I., Chapter III. Simple Periodic Currents, page 101, Second Edition.) If the curve is a simple sine curve the ^mean* value of the ordinate can be shown to be equal to the maximum value divided by N/2. But this rule does not hold good for forms of curve other than the simple sine curve. The measurement of alternating currents is made to depend either upon their heating power or upon ih electro-dynamic attraction between different parts of the same circuit. If a current Hows through a circuit, the rate of heat production depends ( 2 ) on the square of the current strength at that instant. Hence, if the current is periodic, the mean rate of production of heat depends on the mean-square value of the current. If two parts of the same circuit are parallel to each other, and if one is fixed and the other movable, and if a current flows through them, the attraction or repulsion between these parts of the conductor depends on the square of the current strength in them. Hence, if the current is periodic, the mean force or stress between them will depend on the mean-square value of the current. An alternating current of one ampere is thus denned : It is a periodic current such that its v'mean" value is unity, or one which will produce the same heating effect as a continuous current of one ampere would do when passed through the same conductor. The most useful type of instrument for measuring alternating currents is a dynamometer. Of these there are many kinds, such as the Siemens dynamometer, Lord Kelvin's ampere balances, and other similar instruments. In all of them, however, the current flows through a conductor, part of which is fixed and is called the stationary coil or coils, and part of which is movable and placed near the stationary part. The current flowing through the fixed and movable parts exerts a force drawing them together, and this stress is resisted either by the torsion of a wire or spring, or by gravity. If the time of free vibration of the movable part is very large in comparison with the duration of one complete cycle of the current, then the mean stress or force between these two parts is a measure of the mean-square value of the current if this should be periodic in value. The alternating current is measured in such an instrument by observing either the torsion or couple required to bring back the movable part to an assigned position with reference to the fixed coil, or else by the displacement of the movable part as observed on an arbitrary scale. To calibrate a Siemens dynamometer, set it up in series with a standard cali- brated direct current instrument, and send the same steady continuous current through them both. Observe the scale readings of the Siemens instrument for each particular value of the direct current. These same scale readings give the correspondingly valued alternating current. In the Siemens instrument the scale is usually a scale of equal divisions, 400 to the circumference. The number of degrees of torsion required to be given to the spring in order to bring back the movable coil to its normal position is indicated by a pointer moving over this scale. If the degrees of torsion required to bring the movable coil back to the zero position when one ampere is flowing through the coils is known call it n then the number of degrees of torsion required when A amperes is flowing through the coils is n A 2 , and n is called the constant of the instrument. In the case of instruments to be used for measuring alternating currents, large errors are likely to be introduced if any metallic parts are placed near the movable coil, ( 3 ) by reason of the fact that eddy currents are set up in these parts and react upon the movable coil. Hence no metallic frames, cases or supports must be used, and this source of error must be guarded against. The Student should calibrate a few alternating current ammeters, using, if possible, currents of different frequencies. He will then find that many alternating current ammeters with iron in their coils do not give the same readings with different frequencies, and it will be seen that certain types of alternating current ammeter are only useful if calibrated with the current with which they are to be used. Hence the above described process of calibrating with a direct current must be applied with caution. The best method to employ is to use a dynamometer or ampere balance having no iron in the coils at all. Then, in order to ascertain whether there is any sensible action produced by eddy currents set up in the framework of the instrument, pass the strongest alternating current possible through the movable coil, but nothing through the fixed coil. Note if the movable coil is at all displaced. If not, then cali- brate this dynamometer by the ampere balance with direct currents. Then place it in series with the alternating current instrument to be standardized, and pass an alternating current through the two derived from the supply on which the alternating current instrument is to be used. Calibrate the ammeter by reference to the dynamo- meter. In testing an alternating current ammeter examine the following points : (i.) Ascertain if the instrument gives the same reading for alternating cur- rents of different frequency,, but which have the same mean-square value as ascertained by a correct dynamometer. (ii.) If the instrument is a dynamometer, ascertain if there is any deflection of the movable coil when a current flows through it, but none through the fixed coil. (Hi.) If the instrument contains iron, note if the reading of the instrument i,s the same for the same current when reached by ascending from a smaller current as well as when descending from a larger current. (ir.) Try if the instrument is affected by the presence of permanent magnets. The Student should examine in this mariner one or more forms of commercial ammeters for alternating currents. EXAMINATION OF AN ALTERNATING AMMETER. Observation No True value of the current through Scale reading of the instrument Error of the instrument. Frequency of the current. Remarks. instrument. / - These Notes are copyright, atid all rights of reproduction arc reserved. The Notes can be obtained separately or bound complete. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The Electrician" Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LVKOltATOIiY .NOTES AND FORMS. Name. JJo. 3O.~APVA*JCEr> (No. 10). Date delineation of Hlternatino Current (turves. Tin 1 i>i>aratus required for these experiments is an alternator, on the shaft of which /x a revolving contact-maker so arranged as to close a circuit at any assigned instant during the passage 'of the armature coils between the field magnet cores. Other apparatus required includes a set of 50 small .icciniili'i'ii n'lls, an, electrostatic voltmeter, and a condenser of about one- third microfarad capacity. The Student is recommended (<> as set up. carefully the arrangement of the apparatus An alternator of any form can easily have a curve tracer applied to it, by means of \\liidi the form of the alternating current curves can be, drawn. On one end of the shaft of the alternator (see Fig. l)is fitted a gnnmetal disk, which carries an ebonite disk about Kn:. 1. Alternator provided with Curve Tracer. four inches in diameter and half an inch wide. The disk must be turned up on the shall so as to be exactly centred. In the edge of the disk is let in a small slip of steel about one-sixteenth of an inch in width and as long as the disk is wide. Two small insu- lated springs, S S, are arranged on a rocking arm, H, which is centred on a fixed external pivot exactly in line with the centre of the armature shaft. These springs must be placed to press against the edge of the ebonite disk, and are then connected together electrically each time the steel slip passes underneath them both. The brushes, B B, can be rocked over so as to make contact at any angular position with reference to a fixed starting point, the angular displacement being recorded on a circular divided scale G. The arrangement of the spring holder and rocking arm will be best ( 2 ) understood from the diagram in Fig. 1, in which is represented a small alternator provided with such a contact-maker. Assuming that the laboratory is provided with this appliance, a number of inte- resting experiments can be carried out. A pair of insulated wires, W W, are connected to the two insulated contact springs, and another pair of insulated wires are connected to the terminals of the alternator and the four wires brought to the experimental table or laboratory. In the first place, let it be assumed that the alternator gives- a pressure ( \/ mean" value) of 100 volts. To describe the electromotive force curve of the alternator, an electrostatic voltmeter, preferably one of Lord Kelvin's multicellular electrostatic voltmeters, has its terminals short-circuited by a well-insulated condenser of about one-third or one-quarter microfarad capacity, and the combination is then joined across the alter- nator terminals with the contact-maker interposed on one side. The effect of this arrangement is that, when the alternator is running, the contact-maker closes the circuit of the voltmeter at every revolution at an instant depending on the position of the rocking arm. The reading of the voltmeter is then the value of the instan- taneous electromotive force of the alternator at the instant during the complete period corresponding to the position of the brushes. By moving the rocking arm over into various angular positions by the handle H the varying electromotive forces at different instants during the phase can be obtained. Since the electrostatic voltmeter does not read on its scale below a certain pressure of 50 or 60 volts, it is necessary to- introduce a constant electromotive force in addition to that of the alternator to obtain a sufficient deflection for those values of the instantaneous electromotive force which lie below 50 or 60 volts. . This is achieved by inserting in series with the voltmeter a variable number of small secondary cells, the electromotive -force of which is separately measured on the voltmeter, and this value deducted froin the total scale reading when the battery is so used. This blocks up the voltmeter to a false zero, and enables volts to be read by it down to zero. In this manner the value of the instantaneous electromotive force of the alternator can be observed and plotted down on a curve in terms of the angular intervals of one complete period, and such a curve is called an alternating current curve. If it be desired to describe the current curve of the alternator, a non-inductive resistance, of such size as to carry the current comfortably without heating, is inter- posed in the external circuit of the alternator. The voltmeter, batterv, and contact- maker are joined in series across this resistance. The curve of potential difference at the ends of this resistance is then the curve of current of the alternator. In takin^ O the curve of terminal electromotive force of a transformer on the circuit of a hio-h O tension alternator it is necessary to divide up the potential and measure only a fraction of it. For this purpose a non-inductive resistance is joined across the high tension circuits. This resistance must be of such a form that it can be placed across the mains safely and carry a current which is not less than, say, half an ampere. Thus, if the circuit is a 2,000-volt circuit, this resistance should have a value of 4,000 or 5,000 ohms. A connection is made to a point of the resistance, which is one-twentieth of the whole, and potential wires are brought from this point and joined to the extremities of the voltmeter-battery and contact-breaker in series. In this way we can plot out a ( 3 ) curve the ordinates of which are one-twentieth of the value of the potential across the whole main circuit. The reason for stating above that the divided resistance should carry a large fraction of an ampere is that, if the divided resistance is a very high resistance say, 40,000 or 50,000 ohms the current through it is not large compared with the capacity-current of the electrostatic voltmeter ; and under these conditions the voltmeter reading would not be the same fraction of the whole potential difference between the mains that the resistance of the small section of the resistance is to the resistance of the whole. The proof of this fact is somewhat long, but the general nature of the effect can be understood by bearing in mind that electrostatic voltmeters have a small but measurable capacity, and that, although they would not pass any continuous current, they do permit the passage of an alternating current, which is called the capa- city-current of the voltmeter. If the voltmeter is joined across a section of a very high resistance, it sensibly shunts some current and reduces the effective resistance of this section. The curve of primary terminal potential difference of a transformer can, how- ever, be described in the above manner. The curve of primary current can be also obtained by putting in series with the transformer a resistance of such magnitude as to afford a drop of about 100 or 150 volts to measure at a maximum. Having described the curves of electromotive force and current, the curve of induction of the transformer may be obtained by integrating the curve of primary potential difference. A set of curves so drawn for a 10-kilowatt transformer are shown in Fig. 2. For further infor- I . A J8! ". )0 F\ 4 V >? Y 20< . S >Q \ / 100 A * o l r < -^ ( > L V- \ t '/ '3 ft) D 4-1 ^ > 1000 >- | 1 2J | / > .* 100 &. . \ / '3 f 20 >0 '% V ./ 1 4000 30 )0 . \, 7 t 1 30 60 90 120 150 180 210 & k> 270 300 3C 3 Fio. 2. Curves of Current and Electromotive Force of a 10-H.P. Transformer taken off a Kapp Alternator. ination as to the manner of obtaining the curve of induction from the curve of electro- motive force, and for fuller details of the method of instantaneous contacts, the Student is referred to " The Alternate-Current Transformer," by Fleming, Vol. II., Chapter IV. Kr a description of a method of measuring these transformer curves when the alternator is not accessible, the Student is referred to a paper in The. Electrician, 1895, Vol. XXXIV., pages 460, 507. The Student should make a complete study of an alternator and a transformer by this method, and record the observations in the appended Forms, plotting down the curves on squared paper. ( 4 ) DELINEATION OF ALTERNATING CURRENT CURVES Electromotive Force Curve. Ratio of divided resistance used Observation No Scale reading of contact- Voltmeter reading. E.M.F. of battery in series with Corrected instantaneous E.M.F. of Phase angle in degrees. maker. voltmeter. alternator. i Current Curve. Resistance used in circuit ohms. Observation No Scale reading of contact- Voltmeter reading. E.M.F. of battery in series with Corrected instantaneous E.M.F. at terminals _r Corrected instantaneous value of Phase angle in degrees. maker. voltmeter. OI resistance. * These Notes are copyright, and ail rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ?s [UNIVERSITY] ELECTRICAL LABORATORY NOTES AND FORMS. No. 31 ADVANCED (No. 11). Name Date Efficiency {Test of a ^Transformer. 7'/c' i>i>aratus required for these experiments is an alternating current transformer ', and a means of supplying it with alternating current at a pressure of l,00(f or 2,000 volts. The load on the secondary circuit of the transformer must consist of incandescent lamps. The instruments required are an ammeter and voltmeter for reading the primary current and pressure, and other similar instruments for reading the secondary current and pressure, and also a non-inductive ivattmeter. Rheostats and resistances must be provided for regulating the primary pressure. Tin- Student is recommended to sketch out very carefully the arrangement of circuits, and to examine them well before, beginning the test. In these and all other high-tension experiments, it is an ESSENTIAL PRECAUTION to ivear a pair of indiarubber gloves and to stand on an indiarubber mat, to avoid, risk of accident. An alternate current transformer consists of two circuits wound over an iron core, called the primary and secondary circuits. One of these, generally called the primary, is employed to transmit a high pressure alternating current, and the other, called the secondary, lias then produced in it another current at a lower pressure, called the secondary current. When the transformer is at work there is a certain difference of pressure between the primary terminals, which varies in a periodic manner, and if a high- tension electrostatic' voltmeter is joined across these terminals the voltmeter will measure the \/mean* value of this alternating pressure, and this is called the primary potential difference (P.P.D.). In the same way an electrostatic voltmeter placed across the secondary terminals will measure the Vmean" value of the secondary potential difference (S.P.D.). This symbol /tnean* stands for the long phrase " the square root of the mean of the squares of all the instantaneous values of the varying quantity (current or electro- motive force) taken at numerous equidistant intervals during the complete period." If a Siemens dynamometer is inserted in series with the primary circuit of the transformer, we can in like manner measure the v/meair value of the primary current. The transformer tu be tested should have a load of lamps or other non-inductive resist- ance arnini;ed on its secondary circuit, and should have an electrostatic voltmeter, care- fully calibrated, placed across its secondary terminals and one across its primary terminals; also a Siemens dynamometer or current balance in series with its primary and secondary circuits. When the load on the secondary circuit is practically a non-inductive load, the product of the \ mcair value of the secondary terminal potential difference in volts (S.P.D.), and the Vmean 2 value of the secondary current (S.C.), as given by the instru- ments, gives us truly the mean power given out in watts on the secondary external circuit. The primary circuit of the transformer is, however, an inductive circuit, and hence the product of the N/mean 2 values of the primary potential difference (P.P.D.) and primary current (P.O.) does not always, or at least when the transformer is lightly loaded, give us the true mean power given to the transformer. The product of P.P.D. and P.O. is called the apparent power or apparent watts given to the transformer. The real mean power or true watts can only be ascertained by a proper wattmeter constructed as follows : The series coil of the wattmeter must be of wire sufficiently large to carry the full primary current of the transformer, and, if that is considerable, it is best made of stranded wire. The shunt coil of the wattmeter must not have more than five or six turns of wire on it, and this should be thick wire capable of carrying from ten to twenty amperes. The wattmeter should be constructed without any metal parts near the movable coil. The movable coil is to be connected to the secondary circuit of a small transformer which supplies it through a bank of lamps or resistance with current at low pressure. This auxiliary transformer has its primary terminals connected to the primary mains of the transformer under test. In the diagram (Fig. 1) the wattmeter is represented by W, S being the series coil, and Sh the shunt coil. The auxiliary transformer is represented by 1\. Its secondary circuit includes a lamp or lamps L, and the shunt coil Sh. The transformer to be tested is represented by T, and the keys k^ and k. 2 close the primary circuit of this transformer, or else the non-inductive resistance in parallel with it. The wattmeter may be preferably one of the Siemens form with spiral torsion .spring and graduated dial. To standardize this wattmeter it is necessary to provide a non-inductive resistance, which is of a form suitable for putting across the high-tension mains, and it will then absorb a power which must be comparable with the power to be measured. A special form of resistance for this purpose has been designed by Dr. Fleming. This non- inductive resistance must be so arranged that, by means of keys k. 2 or k l; the resistance or the primary circuit of the trans- former can be placed in connection with the wattmeter, the arrangements of circuits being as in the diagram. Having connected up the apparatus properly, a scries of observations should be taken of the power taken up by the transformer and that taken up by the non-inductive resistance, as follows. The non-inductive resistance should be divided into two sections, and one of these sections should be of such current-carrying capacity that it is not sensibly heated by the current which will flow through the whole resistance when placed across the high-tension circuit. This small section of the resistance should have such a magnitude that under these circumstances the fall of volts down it is about 100 volts { Vmean 2 value). Connect the whole resistance across the primary mains and regulate the primary pressure until it has the standard value at which the transformer is to be FIG. 1. ( 3 ) t "si nl. Let the value of the small section of this resistance he r ohms and the value of the whole resistance R ohms. Let i> be the fall of volts down the small section and V that down the whole section. Then V represents in watts the mean value of the r power taken up in the non-inductive resistance when a terminal ^/mean 2 pressure of V volts is applied to it. This preliminary test should be carefully made. The non- inductive resistance is then applied to the wattmeter, and the reading of the wattmeter taken when the primary pressure has the standard value V. Suppose the scale reading of the wattmeter is w, then the true watts corresponding to w is - ' and the value of one scale division of the wattmeter is- - watts. The wattmeter is then switched over w r on to the primary circuit of the transformer, and the wattmeter reading taken. Let it be \V scale divisions ; then the mean power being taken on the primary of the transformer \V V i) is equal to - watts. Let the primary current of the transformer at this instant be wr A! ( vnicair value). The apparent watts taken up by the transformer is AV. The ratio of the true watts to the apparent watts is called the power factor of the transformer. At the same time the Vmean* value of the secondary current should be taken, and also that of the secondary potential difference at the terminals. Let these values be A 1 and V 1 amperes and volts respectively. Then A 1 V 1 is the mean power in watts delivered up by the transformer. The efficiency of the transformer is the ratio of power coining out to power going in, or the efficiency e is given by the fraction A 1 V 1 w r w This efficiency should be taken at various fractions of full output. These results should be plotted down in the form of two curves, one showing the increasing efficiency in terms of the secondary load, and the other showing the total loss in the transformer in terms of the same. The observer should then measure the resistance of the primary and secondary circuits of the transformer and calculate the copper losses at all loads, and should take a particular observation to determine the loss in the transformer when the secondary circuit is open. This last is called the iron loss or core loss of the transformer, and it is found that the total loss in the transformer at any load can be calculated by taking the sum of the iron loss and the total copper losses at that load, allowance being made for any rise in temperature in the transformer circuits. The Student should make in this way a careful study of one or two transformers, and enter the results in the following tables. EFFICIENCY TEST OF A TRANSFORMER. Transformer No. by Nominal power kilowatts. Resistance of primary circuit = ohms at Resistance of secondary circuit = ohms at Transformation ratio Standardizing resistance = ohms. Power taken up at volts in resistance Wattmeter calibration = watts. Observation No. Primary volts. Primary current. Secondary volts. Secondary current. Wattmeter reading. Watts into Watts out of Efficiency. Observation No Primary current. Secondary current. Copper loss in primary. Copper loss in secondary. Iron core loss at no load. Total loss in transformer. Fraction of fuU load. These Notes are copyright, and all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DK. J. A. FLEMING, of University College, London, and are published by "The Klectrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. . 32. ADVANCED (No. 12). Name Date ZTbe Efficiency {Test of an Hlternator. Tli>' plant required for these tests is an alternator and a continuous current motor. .I/.M/ tl>e necessary instruments for reading the currents and pressures of both circuits. A non-inductive resistance capable of absorbing the electrical output of the alternator is required. If the alternator and motor can be put it'n upon the same bedplate, with shafts in line and connected by a flexible coupling, this arrangement is the best. If not, the pulleys of the two UK -lii lies must be connected by a fairly long jlexible belt, with good adhesion. The Sti/y the slip of the belt. The pulley is then replaced un the alternator, and the alternator circuits connected up through an ammeter with the non-inductive resistance which is to take up the power. Across the poles of the alternator is placed a voltmeter. Both these instruments must be calibrated for alternating currents of the frequency to be employed. A continuous-current ammeter is also in the feeding circuit of the motor, and a voltmeter across its terminals ; both these instruments must previously have been cali- brated. The combination having been set at work, we note the current A x in amperes going into the motor. This must include that going to excite the fields. We observe the volts V t across the terminals of the motor. The current A 2 coming out of the alter- nator must be taken, and also the volts V 2 across its terminals. The above- described experiments will enable an answer to be obtained to the question, How much power in watts is given to the pulley of the alternator when A t V t watts are given to the motor ? The product A x V t represents in watts the power given to the driving motor. Suppose we have found by the above-described experiments with the brake that this yields a power P watts on the pulley of the alternator. Then if the alternator is giving out a current A 2 at a pressure V, it is giving out A 2 V 2 watts, and the efficiency of the alter- A V nator is therefore given by p 2 expressed as a percentage. This efficiency should be taken for various loads in the alternator, and a curve should be plotted in which the horizontal distances represent outputs of the alternator, and the vertical ones efficiency of the alternator. At the same time we have to take into account the power required to excite the field-magnets of the alternator. To do this we must obtain the field current a in amperes and the volts v at the terminals of the field circuit, and add the product a v, or watts required to excite the field, to the power given to the pulley of the alternator (that is to P) before calculating the efficiency. Hence the correct efficiency A V of the alternator will be given by the fraction =5-7 expressed as a percentage. The Student should in this manner test a small alternator and obtain the efficiency at various loads and speeds, and enter up the results in the appended form. On the subject of testing alternating current machines, the Student is referred to a very excellent Paper, by Mr. W. M. Mordey, in the "Proceedings of the Institution of Electrical Engineers," Vol. XXII., 1893, p. 110. In this Paper it is shown that large alternators with fixed armatures may be tested by using one part of the armature as a motor, against another part as a dynamo. Various modes of testing large alternators are given. EFFICIENCY TEST OF AN ALTERNATOR. Alternator No. by Armature resistance (warm) = Field resistance = Number of bobbins on armature = N umber of poles on fields = l-'rt>(|uency = Normal speed = . Full output = Exciting current = a = Exciting volts = r = watts. Observation No. Current into motor Volts on terminals of motor Current out of alternator Volts on terminals of alternator Speed in revolutions per minute Efficiency of alternator f. = A,. = V,. = A* = V,. = N. - EFFICIENCY TEST OF AN ALTERNATOR. Observation No. . Current into motor -A,. Volts on terminals of motor Current out of alternator = A 2 . Volts on terminals of alternator -V* Speed in revolutions per minute -N. Efficiency of alternator These Notes are copyright, and all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. Wo. 33 ADVANCED (No. 13). Name Date photometric Examination of an Hue %amp. The apparatus needed for these experiments is an arc lamp with very uniform and regular feed, and in the first instance had better be a continuous current arc lamp. This lamp must be suspended from the ceiling so that it can be raised or lowered and moved horizontally. A plane, mirror must be provided, fixed to a horizontal rod or tube at an angle of 45 degrees, this horizontal tube being capable of revolution round its axis. The remaining apparatus comprises a photometric bench, photometer disc, standard incandescent lamp, and measuring instruments. Tin- Student is recommended to sketch carefully the arrangement of the apparatus. If an arc lamp is set up and worked with continuous current, the positive or crater carbon being uppermost, a very little examination shows that the light is not uniformly distributed. A complete photometric study of an arc lamp for practical purposes involves several investigations. The distribution of light has to be examined. The mean spherical candle-power and corresponding electrical power absorbed has to be determined and the action of the regulating mechanism tested. The first question involved is that of the standard with which the light is to be compared. A slight experience of comparing the light of the arc with that of a gas flame or normal incandescent lamp shows that the difference in colour between the two lights is very marked, and that, when using a Bunsen disc, the unpractised eye is very perplexed to establish any comparison between them at all. It has usually been the custom to evade this difficulty by the use of red or green glass screens, and to measure what is popularly called the "red candles" or "green candles" of the arc. This, however, is practically a useless performance. The best standard to select for comparison is a good incandescent lamp, which is used at about 2 or 2-J- watts per candle, and which under these conditions gives a light more suitable for a comparison standard than that of a glow lamp burning at 3 or 4 watts per candle. The very rapid blackening of the standard lamp can be minimised by employing a very large glass, of such size that the filament is four or five inches away from the glass envelope on all sides. This over-burnt incandescent lamp is kept constantly standardized by reference to a normal standard incandescent lamp or gas-flame standard. The best means of comparison is by a star disc or by Trotter's wedge photometer. In this last form of photometer two pieces of perfectly white card or metal, painted dead white, are set up on the photometer bench, their planes being vertical and meeting at an angle of about 80 degrees or 90 degrees. Tin- vertical angle or meeting edge is pointed in one direction along the photometer bench. In that surface nearest the observer are cut one or more slits or holes. If two lights are placed one on either side of the wedge, it will be easily seen that an observer looking at the wedge will see the front or outer surface of the front card illuminated from one side, say the left-hand light. Looking through the holes in this card, he will see the inner surface of the other card illuminated by the right-hand light. The wedge can so be moved that these surfaces are equally illuminated, and then the hole will hardly be seen at all. The process of obtaining this balance of illuminations is facilitated by swinging the wedge from side to side in slowly diminishing arcs. Whatever form of photometer is adopted, the process of photometry consists in effecting a balance or equality in brightness of two white surfaces, one of which is illuminated by the standard light and the other by the light under test. And in making this estimate, the eye has to discriminate the equality in brightness apart altogether from any outstanding difference in colour. The arrangement being thus made for the photometry of the arc, the apparatus should be set up as follows : The arc lamp should be suspended from the ceiling and be so capable of being moved that the arc can be traversed over a half-circle in a plane perpendicular to the direction in which it is to be photometered. The inclined mirror is set on its axis at 45 degrees at the centre of this circle, and provided with means for measuring the angles through which that axis is rotated ; the axis of the mirror must be in the direction of the central axis of the photometer. The mirror is so set that it can catch the rays sent out from the arc at any angle to the horizon and reflect them along the axes of the photometer, and the angle of reflection from the mirror always remains the same. By traversing the arc round a circle whose centre lies on the axis of the mirror, we can always keep the arc at the same distance from the mirror and reflect the ray at the same angle, and yet send the rays coming from the arc at any angle to the horizon along the horizontal axis of the photometer. The mirror must have its coefficient of reflection at 45 degrees determined as in Elementary Form No. 16. The ray reflected horizontally from the mirror must then be photometered against the glow-lamp standard, care being taken to protect the photometer wedge from receiving any rays from the arc lamp except those which have been reflected from the mirror at a definite angle. By raising and moving the arc and shifting the mirror, the observer can determine the caudle-power of the arc in various directions and angles to the horizontal. The distance of the arc from the photometer-wedge and the distance of the glow lamp standard from the same must be carefully measured. If I is the inten- sity or candle-power of the ray proceeding from the arc in any direction, and if a is the percentage of light reflected from the mirror, then I y^r is the candle-power of the ray after reflection. If i is the candle-power of the glow-lamp, and if D and d are the distances of the arc and glow-lamp respectively from the wedge, then I 100 or D 4 rf 2 ' .D 2 100 Whilst one observer is taking the candle-power of the arc, another observer must take the value of the power taken up in the arc. This is done by placing an ammeter in series with the arc and connecting a voltmeter between the two carbons. ( 3 ) Small clips are provided which connect the voltmeter to the carbons about an inch or t\vo above and below the arc. The ammeter should be so inserted in the circuit that it measures the current going though the arc above and takes no account of that passed by the shunt coil of the arc-lamp mechanism. The intensity of the rays sent off in different directions should be measured for every 10 degrees above and below the horizontal line through the arc, and the results then set out in a photometric diagram of the arc. A centre is taken at A (see Fig. I) to represent the arc and a semicircle, BDC, drawn round it with radii spaced 10 degrees apart. On these radii are set out distances to represent to any scale the illuminating power in that direction in terms of the standard, and a curve, A P N, drawn passing through all these points. This curve will then represent the candle-power of this arc in different direc- tions. The mean spheri- cal candle-power can be obtained from it geometrically as fol- lows : Draw a vertical line, EF, which is a tangent to the semicircle at D, and which is therefore parallel to the vertical line through the arc. From the ends of the radii of the semicircle drawn at intervals of N 10 degrees draw hori- zontal lines ; and, start- ing from the vertical line, set off on these horizontal lines dis- tances equal to the The extremities of these Thus the ordinate SP' ri illuminating power of the arc in those different directions. lines define another curve, as shown in the diagram in Fig. 1. of the curve E P' N' F is equal to the radius A P of the photometric curve A P N. The area included by this curve E P' N' F, and the vertical line E F, must be taken with the planimeter and compared with the area of the circumscribing rectangle E G H F. The mean spherical candle-power of the arc lamp is given by the product of its maximum candle-power in candles, represented by the lines A N or WN', and the ratio of the areas included by the above projected curve and its circumscribing rectangle. ( 4 ) THE PHOTOMETRIC EXAMINATION OF AN ARC LAMP. Arc lamp tested = Observation No C.P. of standard lamp. Distance of standard from Distance of arc lamp from C.P. of arc lamp. Angle to horizontal at which ray is Current through arc in Potential difference of carbons i Power in watts spent disk. disk. being sent out. amperes. in volts. in arc. ' These Notes fire copyright, and till rights of reproduction are reserved. The Notes can be obtained .separatel}' or bound complete. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The E lectrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. JTo. 34 ADVANCED (No. 14). Name, Date Measurement of Simulation anfc ffoiob IReststanee. "i>j>aratus required for these tests is a sensitive movable coil galvanometer, some resistance boxes, and a megohm standard. A set of 200 small ivell- insulated secondary cells must be provided, ivhich are preferably small Lithanode cells. The galvanometer, battery, resistance boxes, and all con- nections and keys must be very highly insulated. A long length, about half-a-mile, of insulated cable should be provided for test, and this should be placed in a tank ofivattr ivith its ends out. The Student should sketch the arrangement of all the connections. Although many methods of testing high resistance have been proposed and are to be found described in electrical text books, there is only one method which yields results which are perfectly satisfactory in practice, and that method is the method of testing insulation resistance directly by the current sent through the insulation by a known electromotive force. Suppose, in the first place, a test has to be made of the insulation resistance in megohms per mile of an insulated cable. The first step is to measure the length of the cable. If this is not already done and certified, the only way is to weigh the whole coil of cable and then weigh a known length, say fifty yards, of the cable, and deduce the whole length of the cable from the ratio of these weights. The ends of the cable must then be trimmed by removing, for a length of about one yard from each end, the cotton or hemp twist, tarred tape, or protective layer, but leaving the indiarubber or gutta percha insulation. These ends must then be carefully dried. The coil, with the excep- tion of these ends, is then immersed in water at 70F. in a tank and left there for 24 hours. The free ends must be tied up by silk strings so that they may not get wet. The copper strand is exposed for an inch at each end, and the two ends of the copper conductor of the cable may be twisted together. The galvanometer must then be set up in a steady place, a divided scale placed at the proper distance, and a very sharp image of an incandescent lamp filament focussed upon it. The battery is then set up and well insulated on slips of ebonite, and a highly insulated key placed in series with it. One end of the battery is then joined to the tank and the other end to the galvanometer. The other end of the galvanometer is joined to the copper of the cable. A key should be inserted between the battery and tank. Begin the experiment by connecting the megohm resistance in place of the cable and tank, that is, join the galvanometer, megohm resistance, and cells in series, and vary the number of cells until a deflection of the galvanometer is obtained which can be read to one per cent. Let the number of cells so used be n. Measure the voltage of these cells with an electrostatic voltmeter, and let it be V volts. Then, if the deflection of the "alvanomrtcr was <1 divisions, a current of ami eres makes a deflection of d divisions 10 of scale when it passes through the galvanometer. Next place the galvanometer, battery and cable as above in series, and, in the first place, use only one or two cells of the battery to make sure that the insulation is riot defective. If it proves to be sound, gradually increase the pressure until a deflection of the galvanometer is obtained, either about as great as that given with the megohm or as great as the whole number of cells at disposal will allow. To be satisfactory, the number of cells should be not less Anally than about 200, so as to give a pressure of 400 volts. If the deflection of the galvanometer is then d^ divisions with V t volts acting through the insulation of the V rf cable, and if this insulation is R megohms, we have R = ^T-J Test the galvanometer and battery for leakage by removing the connection between the battery and the tank, when no galvanometer deflection should be found. It is convenient to short-circuit the galvanometer terminals during the first closing of the battery circuit to avoid the sudden and large ballistic deflection of the galvanometer which takes place at first contact owing to the capacity of the cable. When, however, the cable is charged, it will be found generally that the galvanometer deflection decreases from moment to moment as the time of electrification increases. This is caused by an increase in the dielectric resistance which takes place from moment to moment. It is generally usual to record the insulation resistance after one minute's electrification. With some kinds of dielectric it will be found that the insulation resistance goes on increasing for a very long time, and the student should note this and plot a curve showing the time-increase of dielectric insulation. The insulation resistance also decreases generally as the temperature rises, and hence it must be recorded at a known temperature, which is usually 70F. Useful experiments can be made in the laboratory with about half-a-mile of insulated cable in a tank. The results of measurement are generally reduced to megohms per mile at 70F. after one minute's electrification with a pressure of 400 or 600 volts. In testing house-wiring insulation, it is useless to use less than 100 volts pressure in testing. In such tests the circuit switches of the house should all be closed, so as to connect together all the wiring. and 'the lamps be removed. The insulation resistance of both positive and negative sides should be taken separately. Very various rules have been laid down by different authorities as the standard insulation resistance which should be reached by good house wiring for electric lighting purposes. The Phoenix Fire Office requires a minimum insulation resistance of 12' 5 megohms per lamp for continuous currents, and double this for alternating currents. The Institution of Electrical Engineers recommend leakage not to exceed one five-thousandth part of the total working current. Various electric lighting companies adopt standards of from 10 to 100 megohms per lamp as the insulation resistance required in a building. The state of the atmosphere has, however, a great deal to do with the insulation measurement of a building completely wired and with all fittings on, because no inconsiderable portion of the leakage is not true insulation conduction in the cables, but surface leakage over the porcelain, slate, fibre, or wooden bases of sockets, fuzes, switches, cutouts, &c. The minimum insulation allowable in the cables and wires themselves for 100-volt work is about 300 megohms per mile after 24 hours soaking in water. For high-tension work at 2,000 volts the insulation resistance of cables should be 4,000 or 5,000 megohms per mile at least. For this latter class of work too much stress should not be laid on mere insulation resistance. The only satisfactory test is the actual breaking-down pressure in volts. THE MEASUREMENT OF INSULATION RESISTANCE. Length of cable or circuit tested = Time of soakage in water All tests made after one minute's electrification. I Galvanometer Observation deflection Battery through E.M.F. standard ! in volts, resistance. Resistance used to standardize. Galvanometer deflection through insulation under test. Battery E.M.F. in volts. Calculated insulation resistance Circuit or cable tested. THE MEASUREMENT OF INSULATION RESISTANCE. Galvanometer Galvanometer Observation deflection through Battery E.M.F. Eesistance used to deflection through Battery E.M.F. Calculated insulation Circuit or cable No standard in volts. standardize. insulation in volts. resistance. tested. resistance. under test. These Notes are copyright, and all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by "The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. Mo. 35. APYAyrCEP (No 15) Name ' Date Complete Efficiency ftest of a apparatus required for these tests is a potentiometer set and galvanometer, and also some low resistance standards capable of carrying the full discharge current of the battery. A bank of incandescent lamps as an absorbing resistance must be at hand, on ivhich to run down the battery. An ammeter, voltmeter, and carbon resistance are also necessary. The Stixlent should sketch the arrangement of the apparatus and circuits. The Student will be assumed to have a general acquaintance with the chemical changes taking place in a secondary battery during charge and discharge. When M M'condary battery is charged during a certain time, a current is put into it at varying or constant potential. At any instant a certain current in amperes is being supplied to the battery, which will generally vary from instant to instant. If at any instant the value of the current flowing into the battery is taken by an ammeter, and if also the difference of potential is taken at the same instant by a voltmeter, the product of this current and potential difference in amperes and volts gives us the value of the power in watts being supplied to the battery at that instant. Supposing, then. that a straight line is taken, on which we mark off a length representing the time of charging, and at the proper intervals erect perpendiculars to represent to any scale the ampere-current flowing into the battery, and also the watt-power put into the battery, and join the tips of these perpendiculars by a curve, the area included between the time base line, the extreme perpendiculars, and the curves estimated in square units, representing an ampere-hour or watt-hour, will give us the total quantity and total energy respectively put into the battery. Then let the battery be discharged through a resistance, and the current flowing out of the battery and the potential difference at its terminals be continually noted and recorded. We can, on the same diagram, draw curves representing the discharge of the battery in amperes and watts. The ratio between the ampere-hours taken out of the battery and the ampere-hours put into the battery is the ampere-hour efficiency (A.H.E.) of the battery, and similarly the ratio between the watt-hours taken out of the battery and the watt-hours put into the battery is the watt-hour efficiency (W.H.E.) of the battery. The A.H.E. will vary with the time the battery has been standing, and with the rate of discharge. The greater the mean current taken out of the battery, the less, generally speaking, will be the A.H.E. The W.H.E. varies also with the rate of charge and discharge. In order to charge the battery, the working or external E.M.F. must exceed that of the battery, which is two volts per cell, and it is usual to employ 2' 5 volts per cell to charge. Hence, the W.H.E. does not generally exceed 80 per cent., and is often much less. The Student should experiment upon a single cell or pair of cells. The weight and total exposed surface of the plates, positive and negative, separately and together, should be taken. The weight of the glass or other box and acid should also be taken. The cell should then be completely discharged and the battery joined up as follows : An ammeter and a carbon resistance should be placed in series with the cell, and a voltmeter across its terminals. The process of charging should then be begun and continued steadily until the bubbles of gas begin to come off' freely, when it should be stopped. The cell should then be discharged through the carbon resistance and the volts and amperes of discharge observed. The charge and discharge curves should then be drawn, and the ampere-hours and watt-hours put in and taken out, reckoned up, and the efficiencies obtained. This should be done for several different rates of discharge, keeping the current constant by the adjustable carbon resistance. The total ampere- hours of charge should then be reckoned out per kilogramme or per pound of plates, taking both positive and negative plates together, and the capacity per square foot or square decimeter should also be taken, reckoning positive plate surface only. A curve should be drawn representing the varying decrease of ampere-hour capacity with increasing rate of discharge, and the same for the watt-hour efficiencies. In the case of a large cell or cells, the potentiometer may be used to measure the current and volts in charge and discharge, the battery charging or discharging current being taken through a resistance strip. The weight of the plates should be taken again at the end of all the experiments, to ascertain how much they have lost by disintegration. Note should be taken of any buckling or bending of the plates. During the process of discharge the circuit should be opened at intervals, and the open circuit volts of the cell measured. This will enable the observer to calculate the internal resistance of the cell (see Elementary Form No. 12) corresponding to any given rate of discharge. The discharge should be con- sidered to be over for practical purposes when the open circuit potential difference of the cell falls below 1'9 volts. All discharge beyond that point is useless for working incandescent lamps in parallel. Hence the ampere-hour and watt-hour discharge must be reckoned as complete when this point is reached. The Student should enter up his results on the appended Forms and carefully set out all the results in the form of curves. The Student is referred for much useful information on the process of charge and discharge of a secondary battery to a Paper by Prof. Ayrton in the " Proceedings of the Institution of Electrical Engineers," 1890 ; Vol. XIX., p. 539, entitled "The Working Efficiency of Secondary Cells," and to the important discussion which took place on this Paper. TEST OF A SECONDARY BATTERY. Cell or battery tested = Total weight of plates = Weight of box and acid= Total surface of positive plates CHARGE. DISCHARGE. nk No Ampere current P.D. at terminals Time. Ampere- current P.D. at terminals Time. Internal resistance into cell. in volts. out of cell. in volts. of cell. i i W ^\ ^^ tJJTJ7B!lSIT7] TEST OF A SECONDARY BATTERY. Observation No. CHAKGE. DISCHARGE. Ampere- hours put in. Watt-hours put in. Ampere- hours taken out. Watt-hours taken out. Efficiency. A.H.E. W.H.E. i These Notes are copyright, and all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DK. J. A. FLEMING, of University College, London, and are published by " The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LABORATORY NOTES AND FORMS. Mo. 36. ADVANCED (No. 16). Xxme Date ZTbe Calibration of Electric flbeters. Tin 1 apparatus needed for these tests is an ammeter and voltmeter, which should previously have been standardized ivith the potentiometer, and a variety of electric meters. The supply of current to the laboratory, in order to obtain perfect steadiness of current, must be from secondary batteries. The Student should sketch carefully the arrangement of the apparatus as set up. Electric meters may be broadly divided into ampere-hour meters and watt-hour meters. The former register or reckon up quantity of electricity, and the latter record energy. A Board of Trade Unit (B.T.U.) is a unit of energy equal to 1,000 watt- hours. Many forms of meter register directly in Board of Trade units. Meters may be divided into self-registering meters, in which some counting mechanism records and adds up the ampere-hours or watt-hours passing through the meter, or they may require some operation of weighing or measuring to be performed before the result is known, as, for instance, in Edison's electrolytic meter. Assuming, however, that the meter is a self-registering one, it may be tested in the following manner. Set up the meter in a proper position, either screwed to a wall or table, and connect it to a bank of incandescent lamps, so that it will record the quantity or the energy given to them. Insert an ammeter in series with the lamps, and a voltmeter across the terminals of the lamps. Start the current at a known instant, and observe the current and volts at regular and noted instants of time for a sufficiently pro- longed period. The meter reading should be taken at the beginning and at the end of the run. From the instrumental readings it is possible to plot out a curve of energy or quantity given to the lamps. To do this, take a horizontal line to represent time, and mark off lengths to represent hours. At proper intervals set up perpendiculars repre- senting the current and the walls given to the lamps at that instant, and join the tops of all these lines so as to form a current or power curve. Integrate this curve and obtain the whole area included between the time base line, the curve, and the terminal perpendiculars in terms of a unit representing one ampere-hour or one watt-hour. Compare this observed and calculated value with the meter reading. Try the same fur very different currents, that is to say, do not keep the current constant, but van- it as much as possible. If the meter is one intended only for alternating currents, then the ammeter used must be one suited for the.se currents, such as a Siemens dynamometer or Kelvin ampere balance, and should previously have bei-n carefully Standardized. Likewise the voltinrtn must in this case lie an instrument, sn<-!i as a Cardew or electrostatic voltmeter, suitable for use with alternating currents. In addition to cheeking the accuracy of reading of the meter, several other matters should be examined. The liability of the meter to be dis- turbed by the presence of magnets should be tested. If the meter is of the dynamo- meter type, and has a shunt and series coil, the arrangement of this shunt coil should be examined to .see if it is liable to become short -circuited. Also the amount of power taken upon this shunt coil should be measured by measuring the shunt coil current, and note taken whether supplier or customer pays for this power. A very small loss in this respect may mean a considerable total in the course of a year. Since there are 8,765 hours in a year, even one ivatt wasted hourly all the year round means nearly nine B.T.U., and thus 10 watts means 90 B.T.U., which at 6d. per unit amounts to a total loss of 2. 5s. Another point which should be carefully examined is the starting power of the meter. A good 25-ampere meter should start with at least '3 ampere. The current required to start the meter should always be less than that of one eight-candle lamp. The meter should be tried not only with a varying current, but also with constant currents from the smallest to the highest it will carry, and the meter reading compared with the observed and calculated ampere-hour or watt-hour delivery through it. If the meter contains a clock, the going-rate of this clock should be independently examined. Gene- rally speaking, watt-hour meters are to be preferred for electric lighting work to ampere-hour meters, and meters which will act both for alternating and direct currents to meters which will act but for one kind alone. It is impossible to indicate all the special points which the observer should investigate, as these will depend upon the construction of the meter ; but generally the points to be regarded are not merely accuracy under the laboratory test, but liability to derangement in actual every-day use over long periods. Under this head come such matters as failure of action from tarnished or dirty contacts, mercury cups or platinum points ; gradual increase of friction at bearings ; short circuits in shunt coils, or leakage of current across insulators. A satisfactory test of a meter on these points cannot be conducted in a short time ; nothing less than three months' actual use is sufficient. Some meters are intermittent integrating meters that is to say, the record of the current or power is not made continuously, but the meter takes a reading of the current or power every few minutes and then adds up the result. There is an objection to this class of meter wherever the current is liable to vary very rapidly, and in testing such a meter care should be taken to see that it docs not stick or over read a small current after a much larger one has been passed through it. If the motive power of the meter is an electromotor, or the meter contains electromagnets, the amount of power taken up by these should be ascertained, because, as shown above, a small but continuous absorption of power means a good deal in the course of a year. ( 3 ) THE CALIBRATION OF ELECTRIC METERS. Type of meter tested = Calculated Observation i No Meter reading at start. Current in amperes Potential difference in volts. Time. Meter reading at stopping total ampere- hours or watt-hours Meter error. passed. 1 THE CALIBRATION OF ELECTRIC MKTERS. Observation No Meter reading at start. Current in amperes. Potential difference in volts. Time. Meter reading at stopping. Calculated total ampere- hours or watt-hours Meter error. passed. - - These Notes are copyright, and all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by " The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England, ELECTRICAL LABORATORY NOTES AND FORMS. UTo. 37. ADVANCED (No. 17). Name Date Delineation of Hysteresis Curves of Jron. The apparatus required is a magnetometer set up in a steady place. A long vertical magnetizing coil must be fixed near it, and means provided for regulating, measuring and reversing the current through this coil. The experiment cannot be performed in the neighbourhood of moving iron, or of iron hot- water or steam pipes, as these cause changes in the earth's magnetic farce in their neighbourhood. The Student shoidd sketch the arrangement of the apparatus as set up. If a rod or ring of iron is magnetized in one direction by a magnetizing force which slowly increases, and if at every stage of its increase the induction in the iron is measured, we can plot out a curve of rising induction in terms of magnetizing force. If then, after reaching a certain maximum value of the force, the direction of the force is reversed and it is diminished again and carried back past zero to an equal maximum value -in the opposite direction, and then brought back again to zero, we complete what is called a cycle of magnetization. When the induction is plotted out in terms of the magnetizing force throughout the cycle, after going round the cycle once or twice it will be found that the induction curve is a closed loop. It is clear, therefore, that the induc- tion lags behind the force in phase. This phenomenon is called magnetic hysteresis, and the curve so drawn is called the hysteresis curve of the iron. To delineate the curve we proceed as follows. On a long wooden or paper tube, two circuits of insulated wire are \v< mud to form two magnetizing coils. This double coil should be about four feet long and one inch in internal diameter.and be wound with No. 18 S.W.G. double cotton-covered wire, and about six layers of wire be put on one coil and one on the other. This constitutes the magnetizing coil. This coil must be fixed in a vertical position against a wall running north and south. Against the same wall, at a spot six or eight inches from the coil, and at about one-quarter of its length from the end, is to be fixed a box which contains the magnetometer. This consists of a light concave mirror of silvered glass suspended by a cocoon fibre, and having three or four small magnets of watchspring fixed on the back. These magnets should not be more than a quarter of an inch in length. A lamp and scale must be provided to obtain a sharp image of an illuminated slit on a scale a metre away, and by means of the scale the angular movement of the needle can be determined. A few rods of iron are then provided, about thirty inches long and one-twentieth of an inch in diameter. In order to prevent the current in the coil from influencing the needle directly, a compensating coil must be provided. This consists of insulated wire wound on a ( 2 ) bobbin and placed in series with the magnetizing coil, and so situated with regard to the magnetic needle that the resultant magnetic force of the two coils at the place where the needle is is always in the direction of the undisturbed needle. When this is the case no current sent through the two coils in series can disturb the normal position of the needle. These arrangements being made, one of the iron rods is introduced into the magnetizing coil and a small current sent through the coil. The magnetic needle then takes a deflection. The rod must be moved up or down until this deflection is a maximum. The current in the coil must then be measured by a potentiometer arrangement or by a sensitive ammeter. The experiment then consists in gradually raising the current in the coil and observing at each stage the deflection of the magnetic needle. The current is raised to a maximum value, then lowered again to zero ; next reversed and increased to an equally great negative value, and then brought back to zero. This cycle of. magnetizing force must be repeated half-a-dozen times, and the magnetizing forces and corresponding deflections of the needle observed. If the object is merely to obtain the form of the hysteresis curve, it will be sufficient to plot out these magnetizing currents as the abscissae of a curve, and the corresponding deflections of the magnetometer as ordinates. This will give the form of the hysteresis curve plotted to an arbitrary scale. If, however, the absolute value of the induction and force are required, we have to calibrate the magnetometer. From the known number of turns N on the magnetizing coil, and the dimensions of the same, we can calculate the magnetizing force H due to the current of A amperes in the coil from the formula TT 4*" ampere-turns 10 ' length of coil This force is, however, added to or subtracted from the earth's verticaj magnetic force which exists inside the coil. If a long iron rod, at least 400 diameters long, is magnetised, we may consider that at a point near each end there is a magnetic pole of strength m. The whole number of lines of force coming out from the pole, or the whole induction up the centre of the rod, is equal to ^m units. If the section of the rod is s square centimetres and the length is I centimetres, then the volume is s I cubic centimetres. The intensity of magnetization of the rod being assumed to be uniform and equal to I, we have by the usual equation m I I = i ' Is where m is the strength of each pole and I is the length of the rod. Calling B the induction at the centre of the rod, we have B s = 4?r m. Hence B s = 4ir I s, or B = 4sr I ; B hence m = I s = ^ s. If the upper magnetic pole of the rod is on a level with the magnetometer needle, and if P is the distance from the needle to the pole, and likewise if OP 1 is the distance from the needle to the lower magetic pole of the rod, it is easily seen that the magnetic force due to the two poles of the rod are equal to a force F acting on the magnetic needle, where m TO P F = (OP) 2 " (OP 1 ) 2 OP 1 ( 3 ) If this force, acting on the suspended needle, causes it to make a deflection 6 when F acts at right angles to the earth's horizontal magnetic force H, we have F = H tan 0. Hence, since m = -; s, 4T ' B / 1 OP \ H tan 6 -= * V-Qp - OF*/' and B is determined in terms of quantities which can all be measured easily, knowing the value of the earth's horizontal force. If the rod is at least 400 diameters long, the influence of the poles induced in the rod in weakening the effective magnetizing force will he negligible, but this is not the case if it is a short rod. If, then, the earth's vertical magnetic force V is known, the total magnetizing force M, acting on the long rod, is equal to 4"" ampere-turns 10 length of coil * where the term " length of coil " means the length between the cheeks of the magnetizing spiral. Hence we can calculate and find the absolute values of M and B, and plot the hysteresis curve in absolute units. This experiment can only be well carried out in a room where H and V are both constant and accurately known. Provided, however, these conditions can be fulfilled, we can determine in absolute value the area of the hysteresis loop for the iron used. The object of putting the two coils on the bobbin is to be able to neutralize the earth's vertical force by sending a suitable constant current through one of the coils. The earth's vertical force is known to be neutralized when the iron is left perfectly non-magnetic, by causing an alternating current, gradually decreased in strength to zero, to flow through the other coil, and when, after this process, a slight displacement of the rod does not affect the magnetometer needle. For full details of the magnetometer method the Student should consult Chapter II. of the text-book on " Magnetic Induction in Iron and other Metals," by Prof. J. A. Ewing ("Electrician" Series). THE DELINEATION OF HYSTERESIS CURVES OF IRON. Length of iron rod used = Diameter of iron rod used = Distance of magnetometer needle from scale = d = No. of turns of wire in magnetizing coil = N = Length of magnetizing coil = L = Earth's horizontal force = H = Earth's vertical force = V = Observation No. . Excursion of spot of light of magneto- Tangent of angle of deflection of needle tan Current in amperes flowing in Value of 47T AN To TT = magnetizing Value of H tan 6. Value of V. Calculated value of meter =a>. _ = tan20. coil force due to stevesis Xoss. The apjtaratus required for these experiments is a sensitive electrostatic voltmeter adapted for alternating current measurements. A steady source of alternating currents of various frequencies must be available. Some thin sheet-iron must be made up into bundles, each sheet being separated from the next by thin paper. A Siemens dynamometer is also required. Four magnetizing coils, each wound with two circuits, are required for magnetizing the samples of iron. The Student should carefully sketch the arrangement of the apparatus. If a sample of sheet-iron, as used for transformers, is submitted for test, the simplest method by which to examine it is to make up a choking core with a known weight of the iron. Cut a large number of strips of iron, say one or two inches in width and twelve or eighteen inches long. Divide these strips into four banditti and weigh the iron. Proceed then to paper each strip of iron on one side with a strip of tissue paper pasted on, and make up these strips into bundles, fastening the strips together by binding them with tape, but leaving a couple of inches at each end free. Foul- magnetising coils have then to be wound by winding covered wire on a paper tube with wooden cheeks. Two well-insulated wires should be wound together on each tube. The number of turns on each coil must be known. These coils may be wound with two No. 16 double-cotton covered wires laid on together, and must have, at least, 20 turns per centimetre of length. Each coil is then slipped on to a bundle of strips, and the coils arranged in a square form. The ends of the strips of iron where the bundles meet at the corners must then be interlaid or sandwiched in between each other, and squeezed together with a clamp so as to make a good magnetic joint. We have then a square iron frame formed of laminated iron of known mass, .and wound over with two circuits each having a known number of turns of wire. Measure the length of the magnetic circuit by taking the mean length of the square iron core, and calculate the wire turns per unit of length of the magnetic circuit. Calculate also from the known thickness and width of the iron the cross-aectional area of the iron core at the centre of each bundle. Join in series in the same direction the separate circuits on each bobbin so as to make one complete primary and secondary roil, and connect the primary coil with a source of supply of alternating current through a variable resistance. Siemens dynamometer in circuit with this primary coil, and arrange the electrostatic voltmeter so that it can be connected quickly either to the terminals of the primary or the secondary circuit. Measure the electrical resistance of the primary circuit, and ascertain if there is any leakage between the two circuits. If so, the double-wound coils must be re-insulated. The frequency of the alternating current employed must be known. These arrangements being made, we proceed to measure with the dynamo- meter the primary current, and with the electrostatic voltmeter the potential difference at the terminals of the primary and secondary coils. These readings of course give us the mean square (v'mean 2 ) value of these quantities. They are connected as follows with the hysteresis loss in the iron core : Let (?! be the value at any instant of the primary potential difference (P.P.D.), and e, 2 that of the secondary circuit, and let N t and N 2 be the number of windings on these circuits. Let i be the instantaneous value of the primary current, and b that of the induction in the core. These quantities are related as follows : N,S = Ct , ......... (ii.) where S is the cross sectional area of the core. V From the equations (i.) and (ii.) eliminate S and -= , and we obtain the equation r* t Square this equation and multiply all through by d t, and we get , Nj \ 2 1 1 /N V or e i d t = - r i 2 d t H ei 2 d t ( ^ r l } e? d t. '? > i \ N / ^ ^ / yi' 2/ / The expression on the left-hand of this last equation represents the instan- taneous value of the whole power given to the coils and core, and if we write it 1 1 /N \ 2 1 -, j d f ri 2 fjf /7 2 /7/_ I l I * '///_ T*/7f C t tt/ t- I t \.V V Ol \Ji L ^^ I I C a Lt t ^^ / t (.t- t , 2r 2r\N 2 / 2 we see that the expression on the left hand now represents the loss in the iron core alone. If, instead of taking instantaneous values, we take mean values throughout a complete period, and write E t 2 , E 2 2 , I 2 for the mean values of L\, e z , and i respectively, and H for the mean loss in the core, we obtain the equation 1 /N\ I H = -- E t 2 - - ( ft 1 ) E 2 2 - i r I 2 . 2r 2r\N 2 / 2 The quantities E/, E/, I 2 are all given by the instruments. If N : and N 2 are equal, the equation reduces to The loss iii the core is therefore known when we know the terminal volts (mean value) of the primary and secondary circuits, and the value of the primary current and primary circuit resistance. We require to know, however, the value of the mean induction in the core. It' the supply of current is being furnished by an alternator having an electro- motive force curve, not far from a simple sine curve, then it becomes a very simple matter to calculate the value of the induction. For if the primary electromotive force curve is not very different from a simple sine curve, then the curve of induction will be still more nearly a simple sine curve. Let B be the maximum value of the induction, and let p 2* n, where n is the frequency, then b = B sin p t, lu-nee - =pRcospt, ......... (v.) - and from equation (ii.) e z = N 2 Sj) B cosp t. Hence the maximum value of the secondary electromotive force E 2 J is equal to N 2 S/> B, If B 1 is the x/mean* value of the induction density, \we can calculate the value of B 1 , when we know E 2 , N 2 , S and p, from the equation B' == E 2 The value of the primary current should be so adjusted that B l =3,000 C.G.S. units per square centimeter, this being a very usual value for B 1 in the cores of trans- formers. The primary pressure should therefore be varied, and a number of readings taken of the value of E l5 E 2 , and I, and the loss H in core, and the value of the induction in the core calculated for each set by equations (iv.) and (vi.). These values of H and B should be set out in a curve. From the known weight and volume of the core the hvsteresis loss per pound or per kilogramme of the iron can be calculated. In good iron it should not exceed 0'5 watt per pound. It will be seen that, since in equation (iv.) we have to take the difference of the squares of two quantities, these quantities themselves must be measured with great accuracy. It is essential, therefore, to have a very steady alternating current, and to take the readings with great care ; otherwise a very small percentage error in the measurement of E t and E 2 will make a very large percentage error in the value of (E, 2 E 2 2 ). On the question of hysteresis loss in iron the Student may further consult " The Alternate Current Transformer" (Fleming), Vol. II., page 479 ("Electrician" Series). THE MEASUREMENT OF HYSTERESIS LOSS IN Thickness of iron strips = Total weight of iron = Section of iron core = S = Number of turns on primary coil = N, = Number of turns on secondary coil = N a = Frequency of the current = n Resistance of primary coil = r Value of 2ir n p = IRON. Observation Primary P.D. Secondary P.D. " Primary current Induction Total loss in Hysteresis loss per No ov/mean* \/mean* it/mean* value iron core in watts pound in core -E,. -B, = 1. B 1 . H in watts. These Notes are copyright, rind all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DR. J. A. FLEMING, of University College, London, and are published by " The Electrician " Printing and Publishing Company, Limited, Salisbury Court, Fleet Street, London, England. ELECTRICAL LAIIORATOKY NOTES AND FORMS. . 30. ADVANCED (No. 19). Date Determination of the Capacity of a Concentric Cable. Tin' 'iji/>aratus required for these tests is a length of at least half-a-mile of concentric cnhte, a standard condenser, and a ballistic galvanometer ; discharge keys and an insulated battery are also necessary. is recommended to sketch carefully the arrangement of the circuits and l>aratus. Every electric conductor has three qualities, viz., its electrical resistance, depending on the material and size of the conductor ; its electrical inductance, depending on the form and shape of the conductor and the magnetic permeability of the suirounding conducting material; and its electrical capacity, depending on its form and surface, and the specific inductive capacity of the surrounding dielectric, or non-conductor. If an insulated cable with a single stranded conductor is coiled up and placed in a tank if water it has a certain capacity, but if uncoiled and laid out straight on the ground its capacity would have a different value. In general there is not much practical information to be gained by measuring the capacity of a length of single cable, but iu the case of a concentric cable, which consists of one stranded copper conductor wholly .surrounding another, but separated by a definite thickness of insulation, there is a fixed and constant capacity per unit of length which is independent of the way in which the cable is coiled or laid. Concentric cables are almost exclusively used for the conveyance of alternating currents, because, since one conductor wholly incloses the other, there is no external magnetic field, and, therefore, no possibility of disturbance \>namo*. Tin- j>/'i,i/ required for this test is a pair of similar continuous-current dynamos, preferably shunt machines. These must be bolted doivn on a bedplate u'ith shafts coupled in one line. Resistances must be provided in the fields of each machine. A secondary battery must be available for supplying current to make up the difference of power. Connecting leads must be. taken from the machine which acts as a dynamo to that which acts as a motor. Three voltmeters and three ammeters will be required, or else a potentiometer set. Tin' Stmlt'itt is recommended to sketch carefully the arrangement of the circuits. If a dynamo machine is being tested for efficiency in the ordinary manner, by measuring the power applied to it by a transmission dynamometer and the output by electrical instruments, the error made in determining the efficiency is proportional to the error made in measuring the power supplied. If, however, two identical machines are available, it is possible to apply Dr. Hopkinson's method, by which a greater degree of accuracy may be obtained. The simplest method of doing this is as follows : Let the two identical dynamo machines be bolted down on the same bedplate, and let them have their shafts in one line and coupled together. Let the one be called the dynamo (D), and the other the motor (M). Take leads from the terminals of D and join them to the terminals of M, and insert in the circuit of one of these leads a secondary battery so joined in that its electromotive force aids the electromotive force of the dynamo in rotating the motor in the proper direction to self-excite the coupled dynamo Let the field-magnets of D and the field -magnets of M be excited from the brushes of the dynamo and motor respectively. Insert ammeters in the circuit of the armatures of the motor and dynamo. Call the armature current flowing between the machines Aj. Insert ammeters in the fields of both D and M, and call these currents a t and a z respectively. Insert voltmeters across the terminals of each machine. Call the volts across the brushes of the dynamo V l5 and that across the terminals of the motor V 2 , and provide means for regulating the speed of the machines and their excitation by inserting resistance in the circuit of the field-magnets of both machines. The test is started by varying the number of cells in the secondary battery until the current which passes through the armature circuit, viz. A 1; is that corre- sponding to the full load of the dynamo. The resistances in the fields of the machines are then varied until the machines .-icijiiire their normal speed, and the instruments are then read. The arrangement of the circuits may be understood from the annexed diagram. Since the current which comes out of the dynamo is A t arid the terminal P.D. is Y 1; the power coming out of the dynamo is A l V l watts. If V 3 is the P.D. down the secondary battery, then A t Y s is the power sup- V 2 ) S \ ^ plied by the battery. Since V 2 is the P.D. at terminals of the motor, AjV 2 is the power supplied to the motor. Then we have the Fi e- 1 - following measured powers. The power delivered from the dynamo is A! V x watts. The power delivered to the motor is AI V 2 watts. The difference between these powers, viz., A 1 V 2 AtV^ must be supplied by the secondary battery, and must be equal to A l Y 3 . Hence or = A 1 V 2 -A 1 V 1 , = Y 2 -Y 1 . The power A t V 3 = A x (V 2 VJ is required to supply the power lost in both machines taken together. If we assume that the total losses in both machines are equal, we may arrive at the figure representing the efficiency of each machine as follows : The efficiency e L of the dynamo D is the ratio of the power coming out of it to that going into it. The power coming out of D is A t Y t watts into the external circuit and a x Y! watts into its own field-circuits. The total power given out by the dynamo on the external circuit is, therefore, Aj Y! watts. A V The power put into the dynamo is, therefore, ' 3 + A t The dynamo efficiency is, therefore, A^V, AY A V At v, A t Vi Y 2 2 The power put into the motor is A t V 2 + 2 V 2 watts. The power given out by the motor is, therefore, equal to or is equal to Hence the efficienc)' of the motor is L! Y 2 + i Y : watts. (1) V e,= (2) ( 3 ) If for the moment \ve neglect the exciting currents r^ and 2 in comparison with A,, we see that the efficiency of the dynamo c t becomes 1- (3) and the efficiency c 2 of the motor is <' = - *". '. 2 \ : y and that c,e 2 = '. -. . ......... (4) 9 F l If, instead of assuming that the power supplied by the secondary battery is equally divided between the two machines, we assume that the efficiency of the machine used as a dynamo is equal to the efficiency of the machine used as a motor and call this equal efficiency e, and if we call the power supplied on the shaft common to the two machines P, then we have for the dynamos AV, P ' neglecting the power required to excite ; and for the motor P Ilellre = Vv: (5) Comparing equations (4) and (5), we see that Ve l e t = e. In other words, the efficiency found in the assumption that the machine which functions as a dynamo has the same efficiency as the machine which functions as a motor is the geometrical mean of the several efficiences found by assuming that the losses in the two machines are equal when running thus coupled. To obtain a definite result we must make one of these two suppositions. The above method is the most accurate method of finding the efficiency of a dynamo and motor when they are sufficiently alike to make one of the two above suppositions nearly true ; and it has this advantage : that all the measurements to be made are electrical measurements, and hence can be made very accurately. If a Crompton potentiometer is available, the above readings of currents and volts may be made very accurately by inserting in the field and armature circuits low resistances of such value that, with the normal currents through them, the fall of potential down these resistances is not more than 1'5 volts. Then from the ends of these resistances potential wires are brought to the potentiometer, and the three currents, A l5 a lt and .,. and the two voltages, V t and V 2 , very quickly read in succes- sion. The voltages must, of course, be read with the aid of a divided resistance. The Student should in this way make a test of a pair of coupled machines, obtaining the efficiency of each at various loads, and plotting a curve showing the effii-iem-v as a function of tin- current through the armature. HOPKINSON TEST OF A PAIR OF COUPLED DYNAMOS. Dynamo No. by Motor No. by Number of cells of secondary battery used = TABLE I. Observation No Speed revolutions per Armature current A, Dynamo volts v,. Motor volts v Dynamo field- current Motor field- current Battery volts v minute. flj. a. 2 . \ TABLE II. Observation No ....... Watts pus into dynamo w, Watts taken out Efficiency UOjlVCil UU.C A T of dynamo ! of d y namo W, Watts put into motor W,. Watts taken out of motor Efficiency of motor Current A,. TAesc JVofes are copyright, and all rights of reproduction are reserved. The Notes can be obtained separately or bound complete. They are arranged by DR. J. A. 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The nnripal parts are made to gauge and are intt-i THE SCOTT- SISLING SYSTEM la the best for Lighting Private Hnu-- and for all Instalbitiitns UMjiMriim Accumulatoi'8. Spc< -i;i! l>yiriiuo run ning at Const:mt spn-d diaries all the Cells during hours of Lighting. Sim])!f MmiiiPiilation. J*i>cket List ( Dexcnptive Circular nn Application . Gothic Works, Norwich. Trl.-ar^iliic A.Wrras: "GOTHIC, NORWICH." "X BC" Code. NORWICH SHIPLIGHTEP. glEMENS gROTHERS& (g LIIVIITED, Electrical anb tTeleovaph MANUFACTURERS OF lndia*Rubber, Gutta-Percha & Lead*covered Cables & Wires, DYNAMOS AND ALTERNATORS, Instruments, Batteries, Transformers, Lamps, Ammeters, Voltmeters. CONTRACTORS FOR Submarine Cables, Land Lines, Electric Light, Electric Railways & Transmission of Power CENTRAL^ STATIONS. Offices LONDON: 12, Queen Anne's Gate, WESTMINSTER, S.W. GLASGOW: 261, West George St. NEWCASTLE : 15, Victoria Bldgs., Grainger St. West. MELBOURNE : 46 & 48, Market Street. 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