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ELECTRIC WIRING, FITTINGS, SWITCHES, AND LAMPS. By W. Perren Maycock, M.I.E.E. A Practical Book for Electric Light Engineers, Wiring and Fitting Contractors, Consulting Engineers, Architects, Builders, Wiremen, and Students. The Wiring Rules of the Institution of Electrical Engineers are incorporated in the book. With 360 Illustrations. Crown 8vo, cloth, 6s. WIRELESS TELEGRAPHY and HERTZIAN IVAVES. By S. R. Bottone. With 36 Illustrations. Crown 8vo, cloth, 3s. CENTRAL STATION ELECTRICITY SUP- PliY. By Albert Gay, M.I.E.E., Chief Engineer, and C. H. Yeaman, A.I.E.E., Assistant Engineer to the Islington Vestry. With 200 Illustrations. Crown 8vo, cloth, 10s. 6d. TRANSFORMERS FOR SINGLE AND MULTIPHASB CURRENTS. A Treatise on their Theory, Construction, and Use. By Gisbert Kapp, M.Inst.C.E., M.InstE.E. With 133 Illustrations. Crown 8vo, 6s. ELECTRIC TRANSMISSION OF ENERGY, AND ITS TRANSFORMATION, SUBDIVISION, AND DISTRIBUTION. A Practical Handbook. 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WHITTAKER & CO., White Hart Street, Paternoster Square, LONDON, E.G. 71. oLoqfo-^ /-^ U^u ELECTRIC LIGHTING FOR MAEINE ENGINEEES Digitized by the Internet Archive in 2008 with funding from IVIicrosoft Corporation http://www.archive.org/details/electriclightingOOwalkrich ELECTRIC LIGHTING FOR MARINE ENaiNEERS OB HOJF TO LIGHT A SHIP BY THE ELECTRIC LIGHT AND HOW TO KEEP THE APPARATUS IN ORDER WITH 103 ILLUSTRATIONS By SYDNEY R WALKEE H.I.E.E., M.I.M.E., ASSOC. M.I. C.E., M. AMER. I.E.B. AUTHOR OF "ELECTEICITY IN CUB HOMES AND WORKSHOPS' "HOW TO LIGHT A COLLIERY," ETC LONDON WHITTAKER & CO. 2 White Hart Street, Paternoster Square, E.C. AU Bights Reserved GIFT PREFACE. In the accompanying pages, the Author has endeavoured to unite the experience gained during eight years of actual sea-going work, with that acquired in electrical work during the eighteen years that have elapsed since he left the sea ; and to present in a simple and concise form all that it is necessary for marine engineers to know of the construction and management of electric light apparatus, to enable them to deal with any difiQculties that may arise in connection with the apparatus under their charge. In the whole of the matter here written, the Author has had forcibly present in his mind those scenes he took part in during the time he was at sea, — the working and straining of every part of a ship in a heavy sea- way, the facility with which sea-water and briny vapour penetrate to every part of a ship, the constant vibration of every part of her when the main engines are at work, — and in the advice he has given, he has had always before him the first requirement of every apparatus for use on board ship, viz. that it must work under all conditions and be easy of repair, rather than present a very high so-called efficiency. vH iM845523 viii Preface, The Author would repeat what he said in the preface to Electricity in our Homes and Workshops. This book is in no sense intended to take the place of standard technical works, although it is hoped that many young electrical engineers may find some of its pages useful. It is intended, as far as possible, to lead up to the works of the eminent men who have so ably written upon the higher branches of the subject, and to create that fascinating interest in the science that will not permit of studies being confined to the mere fringe of the subject here dealt with. In conclusion, the Author has endeavoured to cover the whole ground within the scope of the task he set himself; but he trusts, however, that if there are any points which he has not rendered clear, marine engineers and others interested in the work, will kindly write to him, or to the publishers, when he will endeavour to put the matter right to the best of his ability. SYDNEY F. WALKEB. CONTENTS, CHAPTER I. GLOSSARY OF TERMS USED IN THE COURSE OF THE BOOK. PAGE What is Electricity ? — Electro-Motive Force — Electrical Pressure — Tension — Voltage — Difference of Potential — The Volt — Resistance — Specific Resistance — The Ohm — Electric Current —The Ampere— Ohm'sLaw—EMF— The Watt— The Electric Circuit — Magnetism — Electro-Magnetism — Lines of Force — Magnetic Field — Direction of Magnetisation — Insulation : Electric, Magnetic — Induction : between Magnets and Con- ductors, between Conductors — Electro- Static Induction, , 1-19 CHAPTER II. DYNAMO MACHINES. Apparatus required for Electric Light — Driving Engine — The best Engine — Methods of Driving — Belts— Ropes — Parson's Steam Turbine — Spare parts of Dynamo to be taken to Sea — Field-Magnets— Armature — Brushes — Alternate Current Dynamos — Continuous Current Dynamos— Series Wound Dynamo, Shunt Wound Dynamo, Compound Wound Dynamo — Limit of Power — Siemens' Alternate Dynamo — Gramme Dynamo — Tyne Dynamo — Victoria Brush — Starting the Dynamo — Trimming and Setting Brushes — Lead of Brushes — Accumulators, . . . . . . 20-78 ix X Contents. CHAPTER III. CABLES AND BBANCH WIRES. PAOB How the size of Cables and Branch Wires is ruled— Arrangement of Connections of Cables and Branch Wires to feed Lamps in different parts of the Ship — Insurance Companies' Rule for Maximum Current to be carried by Cables — Table of Currents that may be carried by Wires and Cables generally in use — Ohm's Law in reference to the Size of Cable to be used — Numerical example of this — Arrangement of Cables, etc., using the Ship as a return — Insulation of Cables for use on Board Ship — Strains to which Insulation is exposed — Sub- stances suitable for Insulation — Lead Covering and Armour for the outside of Cables condemned — Wood Casing for fixing Cables in Jointing Cables and Branch Wires — Covering Joints — Danger of using Staples for fixing Cables— Danger of imperfect Insulation and careless fixing of Cables — Insula- tion — Resistances — Arrangement of Circuits — Main Switch- boards—Grooved Casing — Moisture in Casing — Covering Joints — Joining Wires — Splicing — Joining Cables — Practical Advice, 74-107 CHAPTER IV. ELECTRIC LAMPS AND THEIR FITTINGS. HoAv the Light is produced — Arc Lamps — Incandescent Lamps — Law of the generation of Heat in the Circuit — Variation of Resistance of Metals, and of Carbon, in the presence of Heat — Construction of Incandescent Lamps — Current required — Blackening of the Glass of Incandescent Lamps in burning — Efi'ect of the Deposit of Carbon on the Light given out by the Lamp — Incandescent Lamps on the Series System — Various Forms of Incandescent Lamps : Bottom Loop, Side Loop, Bottom Capped, Central Collar — Dangers arising from ends of Branch Wires coming into Contact — Loose Contacts — Candle-Power of Lamps — Lamp-Holders : Mr. Swan's Earliest— Faults in Wooden Lamp-Holders— Flexible Cord Conductors for Pendant Lamps— Lamp-Holders for Capped Lamps — Instructions for Reeving Wires through Brackets Contents, xi PAOB that support Lamps— Rigid Lamp Fixtures v. Swinging — Acorn Socket-Holder and Tap Switch— Objections to them — Different Forms of Lamp Brackets — Fittings for between Decks, and for Portable Lamps — Reading Table Lamp . 108-152 CHAPTER V. SWITCHES, FUSES, AND CUT OUTS. Origin of the term Switch — Construction of Switches — The Use of Wood, Slate, and Porcelain for mounting Switches on — Requirements of a good Switch — The "Snap" Action for Switches — Sparking — Loose Handle v. Fixed Handle — Various Forms of Switches : Plug Switch, Bar Contact Switch, Single Break Bar Contact Switch, Double Contact Bar Switch, Radial Bai* Switch, Switches with Contact Bar consisting of a number of bent Brass Plates, Special Locking Switch with Details and Objections, Tumbler Switch, Box Switch, Author's Knockabout Switch, Chopper Switch, Main Switches, Double Pole Switches, Two-Way Switches — Cut Outs : Fuses, Different Forms — Bridge Cut Out— Circular Cut Out — Objections to Fusible Cut Outs — Unreliability — Ceiling Roses — Edison's Screw Plug Cut Out — Table of Current required to Fuse Metals — Electro- Magnetic Cut Outs : Construction and Working, Cunningham's, Others — Mercury Contacts v. Spring Contacts, . , . 153-207 CHAPTER VI. MEASUBING INSTRUMENTS. Two Classes of Instruments — Lineman's Detector Galvanometer : Construction and Use— Ayrton and Perry's Permanent Magnet Ampere and Voltmeters — Ayrton and Perry's Helical Spring Ampere and Voltmeters— Other forms of Ampere and Voltmeters — Advice re noting the appearance of Lamps when properly Burning as a Check on Tests— Arrangement of Switch Board for Two Circuits — Voltmeter working by Expansion of Platinum Wire— How to Find the Indicated Horse- Power required for any Installation of Electric Lights, 208-229 xii Contents. CHAPTER VII. FAULTS, OR CAUSES OF FAILURE, AND HOW TO REilEDY THEM. PAQR How all Failures Arise — Disconnections — Numerical Example— Leakage Faults : Numerical Example — Faults in Dynamo- Faults in Cables — Faults in Lamps and Fittings — How to Discover and Repair Faults— Portable Testing Apparatus — Rule for Testing for Break in the Circuit — Testing for Leakage — Testing for Disconnection in the Armature — Testing with the Full Voltage of the Dynamo — Indications of Leakage — Brush- Holders : Rec[uirements of, Different Ideas on the Subject, .... . 230-270 CHAPTER VIII. THE USE OF THE ELECTRIC LIGHT IN PETROLEUM SHIPS. Explosion on board S.S. Tancarville — Different Stages through which Petroleum Vapour passes — Conditions under which the Electric Light may cause an Explosion — How it may be Prevented — Use of Shunt Wound Dynamo — Why it does not afford Complete Protection — Danger of the Skin of the Ship being connected with the Lighting Service — Double Wiring — Switches and Cut Outs on each Main — Danger of Portable Lamps taking Current from Electric Light Service — Avoidance of Joints where Gaseous Vapour is present — Best Form of Cables to use — Use of Armour outside the Insulation of Cables — General Summary of Precautions, . 271--285 CHAPTER IX. CARGO AND SEARCH LIGHTS. Large Incandescent Lamps for the former — Arrangement of Cables and Connections — Current taken by larg5 Incan- descent Lamps — Ring of Small Lamps : Objections to this — Search Lights : Utility of these — The Arc Light : How Produced — Construction of Arc Lamps — Hand Regulation, best for Search Lights — Reflecting and Refracting Apparatus — Search Lights for the Suez Canal — Arc Lamps and Apparatus in Lighthouses, » . . ^ - . 286-295 LIST OF ILLUSTRATIONS. FIGS. PAGB 1. The Attractions and Repulsions between the Poles of Steel Magnets, ....... 10 2. The Lines of Force passing between the Poles of a Horse- shoe Magnet, ...... 13 3. The Curves taken by the Lines of Force passing between the Poles of a Straight Bar Magnet, . . .13 4. How to find the Direction of Magnetisation of a Bar of Iron excited by an Electric Current, . . .15 5. Crompton Dynamo, on Sliding Rails for taking up the Slack of the Belt, , . . . .23 6. Arrangement of Jockey Pulley, for driving with a Short Belt, 24 7. Parson's Steam -Turbine, with Dynamo attached, . . 25 8. Interior of Parson's Steam-Turbine, , . .27 9. Showing, diagrammatically, the winding of Field-Magnets of Series Wound Dynamo, . . . .51 10. Characteristic Curve of Series Wound Dynamo, . . 52 11. Showing, diagrammatically, the winding of Field-Magnets of Shunt Wound Dynamo, . . . .53 12. Characteristic Curve of Shunt Wound Dynamo, . . 54 13. Winding of Field-Magnets in Compound Shunt Wound Dynamo, ....... 56 14. Characteristic Curve of Compound Wound Dynamo, . 59 15, 16. Siemens' Alternate Current Dynamo, with its Excitor, . 60 17. Old Type of Gramme Continuous Current Dynamo, . 61 18. Latest Form of Siemens' Dynamo, with Driving Engine, . 62 19. Siemens' Dynamo employed at Central Stations on Shore, 63 20. The Holmes' Dynamo, . . . . . -64 xiii xiv List of IllMstrations. FIGS. PAGE 21. Scott and Mountain's "War Office Type Tyne Dynamo, . C6 22. Victoria- Brush Continuous Current Dynamo, with Engine attached, ....... 66 23. Diagram of Connections between Dynamo, Cables, and Lamps, ....... 76 24. Diagram showing Connections between Dynamo, Cables, and Lamps, ...... 76 25. Diagram of Connections between Dynamo, Cables, and Lamps, using the Skin and Body of the Ship as Return, 85 26. Method of connecting Branch Wires to the Skin of the Ship, Beams, etc., . . . . .86 27. Switchboard suitable for use on Board Ship, . . 95 28. Switchboard for use with two Dynamos, . . .96 29. Section of Grooved Casing used for Electric Light Cables, . 97 30. Joint in Solid Wire, ends scarfed, ready for binding, . 101 31. Finished Metallic Joint in Solid Wire, . . .101 32-34. Methods of Jointing Electric Light Wires, . . 103, 104 35. Arrangement of Incandescent Lamps in Series-Parallel, . 116 36. Glow Lamp with Loop Terminals, . . . .119 37. Glow Lamp with Brass Collar, . . . .122 38. Incandescent Lamp, with Central Plate and Collar Ter- minals, ....... 124 39. Edison Lamp with Acorn Terminal, . . . 126 40. Edison-Swan "Sunlight" Lamp, . . , . . 128 41. Lug Lamp-Holder, . . . . . . 129 42. Focussing Glow Lamp, ..... 130 43. New Edison-Swan Lamp for Bow and Masthead Lights, . 131 44. Ornamental Glow Lamp, ..... 133 45. Lamp with Two Filaments in Series, for High Voltages, . 134 46. Original Swan Lamp-Holder, for Looped Lamps, . . 135 47. Stronger Form of Lamp-Holder, for Looped Lamps, . 136 48. Lamp-Holder for Capped Lamps, . . . .136 49. Metal Lamp-Holder, for Pendant Capped Lamps, . . 140 50. Section of Central Contact, Collar Lamp-Holder, , . 140 51. Acorn Socket- Holder, . . . . .142 62. Acorn Lamp-Holder with Tap Switch, . . . 144 53. Form of Holder now used with High Candle -Power Lamps, ....... 145 54. Curved Ornamental Bracket, for use in the Saloon, wi+Ji Ground Glass Shade, . . . . .147 List of Illustrations. xv FIGS, PACK 55. Bronze Bracket, with Shade carried at Right Angles to the Base, without Shade, . . . , .147 56. Ornamental Bronze Bracket, with Shade Carrier for Over- head ; generally used in the Saloon for Lamps fixed over the Table, ...... 147 57. Plain Carved Bronze Bracket, with Shade Carrier, . 147 58. 'Tween Decks Guarded Fittings, . . . .148 59. Bulkhead Fitting, ...... 149 60. Bulkhead Fitting for Lighting two Cabins with one Lamp, 150 61. Guarded Portable Lamp Fitting, . . . .150 62. Reading-Table Lamp, . . . . .151 63. Plug Switch, . . . . . .164 64. Simple Form of Bar Contact Switch, . . . 165 65. Another Form of Double Contact Switch, with Central Contact Bar, . . . . . .167 66. Switch similar to Fig. 65, but with Single Contact and Radial Contact Bar, . . . . .169 67. Another Form of Double Contact Switch, with Pull-off Spring, Mounted on Slate Base. The Contact Bar is in the Position of "Light Out," . . . .171 68. The same as Fig. 67, but the Contact Bar is in the Position of "Light On," . . . . . 172 69. Internal View of Tumbler Switch, . . . .174 70. Tumbler Switch, . . . . . .175 71. American "Chopper" Switch, with Fuse attached. The Switch is shown " Closed," , . . .179 72. Author's Main Switch — Bent Spring Pattern, . .180 73. English Modification of the " Chopper " Switch, . . 181 74. Double Pole Main Switch, . . . . .183 75. Author's Two-Way Main Switch — Bent Spring Type, . 185 76, 77. Showing the Simplest Form of Circular Cut Out. Fig. 76 shows the Complete Apparatus and Fig. 77 the same with Cover removed, . . . . .190 78. Form of " Bridge " Cut Out, with Cover, . . .191 79. Another Form of " Bridge " Cut Out, with Cover, . . 192 80. Form of "Bridge" Cut Out, without Cover, . . 193 81. Porcelain Ceiling Rose with Arrangements for Fusible Cut Out, .194 82. Another Form of Porcelain Ceiling Rose with Fusible Cut Out 195 xvi List of Illustrations, FIGS. PAGB 83, 84. Edison's Fusible Cut Out, with the Screwed Plug in which he places his Fuse Wire, . . . . .196 85. Fusible Wire with Clips for replacing readily in Fusible Cut Outs as Figs. 78 and 79, . . . . 197 86. Cunyngham's Electro-Magnetic Cut Out, . . .199 87. Author's Form of Electro-Magnetic Cut Out, . . 201 88. Lineman's Detector Galvanometer, . . . . 209 89. Outside of one of Ayrton & Perry's Ampere or Voltmeters, 211 90. Inside of one of Ayrton & Perry's Magnet Ampere or Volt- meters, ....... 212 91. Inside and Outside of one of Ayrton & Perry's Helical Spring Amperes or Voltmeters, . . . .215 92. Inside View of a form of Ampere and Voltmeter much in use, in which a Steel Magnet Needle is used, . .216 93. Inside of Biirgin's Ampere and Voltmeter, . . . 217 94. Inside View of Apparatus of another class of Ampere and Voltmeter much in use, in which a bent piece of Iron does the work, ...... 218 95. Edison-Swan Ampere Meter, .... 219 96. Cardew's Hot Wire Voltmeter, .... 224 97. Winding of Field-Magnets as they should be, . . 250 98. Windings of Field-Magnets as they should not be, . 251 99. How to Test for a Break in the Circuit, . . .256 100. Outside View of the Author's Testing Apparatus, . .259 101. Plan of the Inside of the Author's Testing Apparatus, . 260 102. Arrangement for Testing Cables for Leakage, . . 264 103. Arrangement for Testing Cables for Leakage, using the Dynamo Current, . , . . .266 ELECTRIC LIGHTING FOR MARINE ENGINEERS : Oft, //OIV TO LIGHT A SHIP BY ELECTRIC LIGHT AND HOW TO KEEP THE APPARATUS IN ORDER, CHAPTEK I. GLOSSARY OF TERMS USED IN THE COURSE OF THE BOOK. Wha-T is Electricity ? The author would advise marine engineers not to trouble themselves very much about this, but rather to make themselves masters of all that can be done by the aid of electric currents, and of all that will happen, whether we wish it or no/ whenever the conditions present are such that an electric current can pass. So far as we know at present electricity is closely analogous to heat, light, sound, and the other physical forces. As with all these, work must be done upon some body, or by some body, before electricity is generated ; and, as with those forces, whenever work is done in the particular manner favour- able to the generation of electricity, then we have all the phenomena attendant on its presence. Properly speaking we do not generate electricity, we transform some other physical force, such as heat, into A 2 Electric Lighting for Marine Engineers. electricity ; and the latter, in expending its energy, becomes retransformed into magnetism, heat, light, sound, or mechanical force, according to the conditions present. It is the province of the electrical engineer to so arrange his apparatus that these transformations occur in the order and in the manner that are necessary for the doing of the particular work he has in hand ; and it will be the author's endeavour, in the following pages, to enable marine engineers to do the same with the electric lighting apparatus under their charge. Properly speaking, too, we do not generate electricity, we create an Electro-motive Force — that is to say, the ability to do work electrically, provided the other conditions are favourable. Electro-motive Eorce is the term used by electrical engineers to denote that a certain power exists between two points, that can be used to drive an electric current through any conductor that may be connected to them. It is used in precisely the same way that marine and mechanical engineers use the term pressure, when deal- ing with work done by steam or water. In fact, the term Electrical Pressure is now often used in place of electro-motive force. Tension, voltage, difference of potential, are also terms that are used to denote the same thing, the latter mean- ing really difference of electro-motive force, and being a relic of the very early days of electrical science, when the mass of the earth was supposed to be zero of electrical potential, instead of being, as we now know, a conductor subject to the same laws as other conductors. The Volt is the unit of electro-motive force. Electrical engineers talk of so many volts at the terminals of the dynamo, between the main cables, or between the cable and the Glossary of Terms used, 3 skin of the ship, where the latter is used as a return conductor, just as marine engineers talk of so many pounds pressure in the steam chest, the cylinder, etc. And, further, the terms are closely analogous, in that voltage and pressure are both lower after work has been done, and by reason of its being done. The pressure at the exhaust port of a steam-engine is considerably less than at the entry port, and the voltage between electric light mains is less after supplying a batch of lamps, or even after the current has passed through a length of cable. Kesistance is the quality possessed in varying degrees by all known substances, in virtue of which electro- motive force is necessary. It is somewhat analogous to inertia in mechanics. The resistance of different substances varies very much. The metals offer very little resistance indeed, in proportion to such substances as porcelain, glass, indiarubber, silk, cotton, etc. ; and hence it was the custom for a long time, and is even now to a limited extent, to divide substances into two classes — conductors and insulators, the former being supposed to allow of the passage of electric currents through them more or less readily, while the latter did not allow of it at all. There is, however, no such dis- tinction in practical electrical engineering. There are only good and bad conductors. All bodies conduct. Even the so-called insulating coverings of cables, allow of the passage of very weak currents, under the strain of the low voltages generally used on board ship, and, as will be seen later on, this small current in time may lead to the complete destruction of the covering of the cable, just as the continuous action of sea -water upon the shell of the boiler gradually destroys it. And, 4 Electric Lighting for Marifie Engineers, further, most; of the insulating covering, which stands well, perhaps for many years, with from 50 volts to 100 volts, the pr«3ssures used at present in ship lighting, would probably break down in a few minutes under the strain of the high tensions (6000 to 10,000 volts) that are (;o be us(jd in connection with the large central station at Deptford. Tlie metals also vary amongst themselves in their C/ONDUCTiNG Power, or Specific Eesistance, as it is termed ; that is to say, the resistance offered by a given volume of each substance as compared with the standard. Silver and (jopper offer least resistance, or are the best conductors, there being very little difference between them. Iron and stecjl have six to seven times the lesistance of copper for the same dimensions. Lead offers twelve times, German silver twelve, carbon 1500 to 40,000, ^Iphuric acid 100,000, water 1,000,000, dry air an infinite number of times the resistance of copper. The resistance offered by any body varies also with it;s dimensions. The longer a wire of a given size is, for instance, the greater for<3e is required to drive a given current through it ; and the smaller a wire of a given length is, tlie gr(;ater resistance it offers. Just as a steam or wat%r-pipe offers a resistance to the passage of the steam or water inversely in proportion to its diamtjter, and directly as its length, so the resistance off(jred by any k>dy to the passage of electric currents varies directly as its length, and inv(jrsely as the area of i1;s cross section. Tlie unit of resistance is called the Ohm, after the celebrated German professor who discovered the very useful law, also called after him, to which electrical Glossary of Terfns used, 5 engineers owe so much. It is fche resistance of about one mile of No. 4 copper wire, or half a mile of No. 8 copper wire. It should also be mentioned that the i'2 Electric Lighting for Marine Engineers, Other Methods of Driving. Where there is room, or it is desired to reduce the first cost of the plant, the dynamo may be driven by leather or otlier belts, by ropes, or by friction. Of all belts, link leather is the best for driving dynamos ; it is so much more flexible, presents no uneven joints, and is so much more readily taken up when re- quired, or spliced when broken. If a belt is used to drive the dynamo, the pulley of the latter should be provided with a wide flange to prevent the belt coming off. The dynamo should also be mounted on sliding rails, as shown in Fig. 5, which ai'e fitted with adjusting screws, as shown, to enable any slack in the belt to be taken up. The principal difficulty in driving by means of belts, on board ship, is the short length available for driving, giving the belt a very small grip on the pulley. This trouble may be overcome by placing a loose pulley near the path of the slack portion of the belt, in such a position that the belt, in passing over the loose pulley, is caused to embrace a larger arc of the circum- ference of the pulley on the dynamo. This arrangement, which is shown in Fig. 6, is called a Jockey pulley. For rope driving, which the author has not seen much used on board ship, but which is used a good deal on shore, both the driving pulleys on the engine and dynamo are grooved, the grooves being of sufficient depth and diameter to receive cotton ropes of a given size. These ropes are usually from 3" to 4" in circum- ference, and their number depends on the work to be Dynamo Machines, 23 Fig. 5.— Showing Cromptoii Dynamo, on sliding rails for taking up the slack of the belt. 24 Electric Lighting for Marine Engineers. done, the number of grooves on the two pulleys corre- sponding of course to the number of ropes required. It is now usual to have as many separate ropes as there are grooves in the pulleys, each rope being spliced so as to form a complete driving strap by itself. One great objection to this plan, however, is the difficulty that is experienced in getting all the ropes equally taut, so that all may bear the same strain. Fig. 6. — Showing arrangement of Jockey Pulley, for driving with a short Belt. To obviate this, a single rope has been used in a few cases, each end being made fast, and the rope passing round both pulleys as many times as required. Parson's Steam-Turbine. As an instance of extremes in high speeds. Parson's steam-turbine, shown in Pig. 7, with its dynamo, which is specially constructed to run at the speed of the turbine, may be mentioned. It runs at from 4000 to 13,000 revolutions per minute, and consists of a series of small turbine or fan wheels, arranged on one shaft ; and in some forms of the apparatus, of varying size. Dynamo Machines, 25 ^ o i (=1 >» 26 Electric Lighting for Marine Engineers. increasing from the point where the steam enters, so as to act as a compound engine does. The steam enters at the centre of the length of the turbine and exhausts, either into the air, or into a condenser, at each end. Fig. 8 shows a section of the interior of the turbine. It was a complaint in the early days of the steam- turbine, that it consumed a good deal of steam. In recent years, however, considerable improvements have been made, as reports of Professor Ewing of Cambridge and Professor Alexander B. Kennedy of London show. By the addition of a superheater, it is stated that the apparatus becomes more economical than the ordinary cylinder engine. One point that must be very carefully borne in mind, in using Parson's steam-turbine or any other very high- speed apparatus, is that all parts where electric currents are taken off should be kept scrupulously clean, for a very slight film of dirt, or oil, may introduce sufficient resistance into the circuit to cause serious trouble. Choose, then, a simple engine and a simple dynamo ; such as engineers can master and do any repairs to, short of actual re-making. Do not be led away by so-called high efficiencies, requiring apparently less steam, if the high efficiency is purchased at a sacrifice of either simplicity or strength ; 5 per cent, or even 1 per cent, of the horse- power absorbed in the electric light engine of even a very large ship is very small in comparison with the total horse- power used ; but it takes a lot of " notions " to produce. Always have spare working parts for engine and dynamo on board ; and let the engineers be thoroughly conversant with the method of changing them. Dynamo Machines. 27 28 Electric Lighting for Marine Engineers. Spare parts for the engine need not be detailed. For the dynamo : spare armature complete, with commutator, and, if the voyages be long, even two — or a second com- mutator ; but if the latter be taken the engineer should see one fitted, and thoroughly understand the arrange- ment of the wires, otherwise he may burn out some of the coils when he starts the dynamo after renewing his commutator. Spare brush holders and a good supply of brushes should also be taken, and a spare set of bearings. The Dynamo. Before going further it may be well to explain, briefly, the construction of a dynamo machine in its various forms. A dynamo electric machine is an apparatus for con- verting mechanical energy into electrical energy in the form of an electric current, and for converting electrical energy into mechanical energy. The latter need not be considered here further than to note that every dynamo which, when driven mechanically, will furnish an electric current, will, when fed with current, give out mechanical power; but that certain differences are usually made in construction between machines designed to act as generators and those designed to act as motors. In every practical dynamo there are two principal parts — the Field-Magnets and the Armature. Both are electro-magnets — that is, bars or other masses of iron, around which coils of insulated copper wire have been wound. The field-magnets are usually bars or slabs of iron held in a frame which forms the body of the machine, and wrapped with cotton-covered wire. Dynamo Machines, 29 The armature consists of either a number of iron discs or a coil of iron wire. In either case the iron is held on some convenient form of driving arrangement, such as a spider with a central boss, keyed to a spindle ; it is carefully insulated by wrappings of calico or other material, and is then wound, as it is termed, with a series of coils of cotton or silk-covered copper wire. These coils are connected to each other, just as if the whole of the coils had been one continuous winding, without break, from one length of wire. The junctions of the coils are connected to separate bars of a cylinder of copper and asbestos or mica, which is also held on the spindle, and revolves with but is insulated from it. The cylinder of copper and asbestos or mica, is called m^ommutator, and its office is to arrange the currents that are generated in the coils of the armature all in one direction, with reference to the wires, lamps, etc., outside the machine. As the armature revolves, currents are generated in all its coils so long as there is a magnetic field provided for it, in the space in which it revolves ; or, as electrical engineers express it, so long as there are lines of force passing through the iron core of the armature. The currents which are generated in the copper coils of the armature are in opposite directions in its two halves ; and if no arrangement is made for collecting them at the points where the current is reversed they simply neutralise each other, no current being available. In all direct current dynamos, however, — those in which the current furnished is always in one direction, — collectors, known as brushes, are placed at these points, and by 30 Electric Lighting for Marine Engineers, their means the two currents that are being generated in the two halves of the armature find an outlet to the cables, lamps, etc. The brushes consist either of a number of small copper wires, usually tinned, soldered together at one end, a number of thin copper plates soldered together at one end, or a piece of wire gauze made up into the required form and sewn. In either case they are held in some form of shoe, which is attached to the frame of the machine. The brushes are held against the revolving commutator either by the tension of a spring or by a screw provided for the purpose. It should perhaps be mentioned that the reason why currents opposite in direction are generated in the opposite halves of the armature is : the coils in the two halves are under the influence of magnetic poles of opposite name ; and that in nearly all dynamos, whatever may be their form, the windings of the wires on the field-magnets are always so arranged that pole pieces facing each other are of opposite name. This is important to remember, as not knowing it has sometimes led to a troublesome failure ; the dynamo apparently not being able to furnish a current. The pole pieces are the iron extensions form- ing the space in which the armature revolves. Alternate Current Dynamos. In what are known as alternate current dynamos, viz., those in which the current is reversed several times a second, and is delivered to the lamps in that form — the construction, though the same in principle, is somewhat different to that of machines designed for direct currents. Dynamo Machines. 31 In these dynamos the field-magnets consist usually of a number of short bars of iron, wrapped with cotton- covered copper wire, and held in two frames forming part of the body of the machine. The two frames, with their two rows of short field-magnets, each row arranged in a circle, face each other vertically, leaving a small space between them, in which the armature revolves ; the bearings for its spindle being supported by the same two frames which carry the field-magnets. The latter are so arranged that on each frame north and south poles alternate with each other, and in the opposing frames north faces south and south north all the way round. The coils of the armature of the alternator are made flat, and often of copper ribbon, the layers being insulated from each other, the edges of the ribbon facing the field-magnets, and the complete armature usually forming a disc or star. In this form of dynamo, the armature coils, whether of ribbon or wire, form one continuous length, their two ends being connected to two insulated brass collars, carried by the spindle, and known as the commutator, though it is really only a collector. The sectional commutator of the direct current dynamo not only acts as a collector, but it also arranges, commutes, or changes the direction of the currents generated in the coils ; that of the alternator merely serves to conveniently pass on the currents to the cables, just as they are generated. In some alternate current dynamos, however, two or three pairs of collars are pro- vided ; the armature coils being divided into as many sections as there are pairs of collars. The object of this arrangement is to enable separate circuits to be worked from one machine without special arrangements. Before 32 Electric Lighting for Marine Engineers. the advent of the compound-wound continuous current dynamo this was a decided advantage, as it facilitated distribution, and consequently there were, until lately, several of these machines running in some of those vessels that were the first to be fitted with electric light. It should also perhaps be mentioned that some of the latest types of alternate current dynamos present points of considerable difference from the above description, but these have been designed for use in town lighting. In order to understand the reason for the use of the alternate current dynamo, it is necessary to go back to the very early days of the electric light. As is well known the arc lamp was the only form of electric light available up to about sixteen years since ; and the first arc lamps in really practical use were in lighthouses, that at Dungeness having been at work ever since about the year 1856. Now, as marine engineers know, the lanaps used in marine lights have two especial requirements. They must be quite steady, and they must maintain the source of light in the focus of the lens or reflector they are used with. In the arc lamp, as the carbons burn away, the position of the source of light is constantly changing, unless the carbons are moved towards each other as they are burnt away. The continuous current, in passing through the arc, consumes the positive carbon, the one from which the current passes, twice as fast as the negative carbon ; while the alternate current con- sumes both carbons about alike, the current, of course passing first from one carbon and then from the other. In addition to this, with the continuous current, the ends of the carbons which form the arc assume different forms as they bum ; the positive becoming a sort of blunt point Dynamo Machines. 33 ending in a little crater, and the negative a cone, with its apex in the arc. The position of the crater and the form of the positive carbon also change very much, so that, as the crater forms a sort of reflector for the arc, it is difficult to maintain the source of light in the focus for long if the continuous current is used. It will easily be understood, therefore, that the problem of constructing a lamp to consume the carbons that shall maintain the arc in one position, is very much easier of solution with the alternating current than with the continuous. Many years after the Dungeness Lights, and some of those on the French coast, were put in, the continuous current dynamo, unless it was constructed with permanent steel magnets, or its magnets were excited by a current of constant strength from another dynamo, was very trouble- some indeed, because the exciting power of its field- magnets, and therefore the EMF developed, was con- stantly changing, and this, of course, would affect both the steadiness of the light and the position of the arc with reference to the focus of its lens, unless the regulating mechanism of the lamp followed every one of these variations, which, of course, it never did. In fact, when, some years after, large electric lights began to be used at dock gates and other places, with series wound continuous current dynamos, those that were successful owed their success to the engine that was used to drive the dynamo. If the engine followed the variations of the arc, running fast when the arc burnt long, that is, the distance between the carbons became long, and going slow when the arc burnt short, as some good-natured engines did, the lamp would burn for hours with only an occasional splutter. Up till very recently there were c 34 Electric Lighting for Marine Engineers, some of those old lamps still burning and doing good service. The first really practical development of electric light- ing in later years was at the Paris Exhibition of 1878, when M. Paul Jablockoff exhibited his then celebrated electric candle, with which the Avenue de 1' Opera was for some time illuminated, and afterwards our own Thames Embankment. The Jablockoff electric candle, and all its imitators and would-be supplanters, were worked with alternating currents ; and the reason was the same as that given for its use in lighthouse arc lamps, viz. the fact that while it consumed both carbons equally, the continuous current consumed one twice as fast as the other. The Jablockoff candle consisted of two thin pencils of carbon, held in a framework, and cemented to each other with plaster of Paris, which is a material of very high resistance. A small pellet of carbon connected the two at starting, for the purpose of allowing the current to pass ; but as soon as that was burnt, the only path for the current was through the arc, across the plaster of Paris, which was consumed as the carbons burnt down. If one of them had burnt quicker than the other, as it would have done with the continuous current, the lamp must very quickly have gone out, as the carbon ends would have been too far apart for the available EMF to maintain the arc. This, and the introduction of other lamps at the same time using the alternating current, revived the use of alternate current dynamos just as the introduction of the transformer system has done at the present day. It was even thought, in those days, that continuous current Dynmno Machines, -ic dynamos were doomed ; and the leading French electrical engineer of the day, M. le Comte du Moncel, in report- ing on the merits of a then recently-invented dynamo, expressly mentioned, as one of its good points, that it furnished alternating currents. Later, when the incandescent lamp was invented, it was found that alternate currents could be used with them equally as well as continuous currents ; the life of the lamp being, if anything, rather longer when the former were used. And, as in those days, there were no self- regulating dynamos, the ease with which separate circuits could be taken from alternate current armatures, so that the lights turned in or out on one branch did not materially affect the burning of those on the other branches, was a great advantage, and led to the further use of the alternate current. Later on, again, Mr. Ferranti and others were able to generate sufficient current for a very large number of lamps from a com- paratively small dynamo. For all these reasons the alternate current dynamo has had a sort of spasmodic life, now being the dynamo 'par eaxellencey now out of date. Then, as to the reasons for having so many sets of magnets and so many poles. The reason is, of course, to produce a good many reversals of current in a given time; but the necessity for this is not immediately apparent. In order to understand it we must dip into another branch of physics, viz. Heat. It has already been explained that electric currents generate heat when they are opposed, or, as electricians express it, when a resistance is offered to their passage through any body 36 Electric Lighting for Marine Engineers. or bodies. It will readily be understood that time plays an important part in the resultant heating effect. Bodies take a sensible time to become heated to a given tempera- ture, both because heat conduction takes a sensible time, and because a portion of the heat generated or trans- mitted is transferred to surrounding bodies by radiation and convection. As engineers well know, the tempera- ture which any body assumes in the presence of a source of heat, depends upon the balance which exists between the heat received from the source and the heat given off to surrounding bodies. If more is received than is given off the temperature rises, and vice versd. Further, the time during which a body is exposed to a source of heat naturally has its effect upon the temperature the body assumes; since, if it is receiving more heat per second or per hundredth or thousandth of a second than it is dissipating, the longer the source acts the higher the temperature rises. Now, an alternating current is to a conductor an intermittent source of heat. Its current commences very weak, gradually increases to a maximum, gradually falls to zero, and then gradually increases again in the opposite direction. It will be evident that if these pulsations are very slow indeed, it will be possible for the conductor to have partially cooled between the times of maximum positive and maximum negative, more particularly if the conductor is so situated that the losses by radiation and convection are large. To obviate this, therefore, the reversals are made of great frequency, many thousands per minute ; so that, whether the heat produced be in the arc or incandescent lamp, or for the high temperatures required for welding by electricity, Dynamo Machines, ^j the conductor never has time to lose the heating effect of one current before it receives another. The reversals, as already explained, are produced by causing the armature to rotate between pairs of electro-magnets whose poles are arranged alternately — first north on one side and south on the other, then south on the side where north was, and north on the other side, and so on; the result of these reversals of the direction of magnetisa- tion of the field being to reverse the direction of the currents generated in the coils of the armature exposed to it. But there are continuous current dynamos that have more than one pair of electro-magnets, and more than one pair of poles for the armature to rotate between, notably the Victoria, — a design of Mr. Mordey's, modified from the Schiickert, the Giilcher, and others. The reason for these designs are as follows : — Tn the early days of the Gramme dynamo, the earliest practical machine in the market, it was thought that the wires which passed inside the iron ring, being, of course, continuations of those on the outside, did no useful work, and might even be creating an EMF of their own in opposition to the working EMF of the dynamo. To obviate this Herr Schiickert of Nurnberg built a dynamo in which the ring was flattened into the form of a disc, but of larger diameter for a given-sized machine than the Gramme ring it was to improve ; and he made his pole pieces into parallel plates, forming a sort of shoe, in which his disc armature revolved. It was subsequently discovered, however, that Herr Schiickert had actually created in his improved dynamo the very fault he had suspected in the Gramme, though in another ;^8 Electric Lighting for Marine Engineers, form. While all the coils of the Gramme ring, no matter what their position, were creating an EMF which added to the total EMF furnished by the armature as a whole ; in the Schuckert, the coils, during a portion of each revolution, were acting against each other, and it became necessary to reduce the sector of the disc embraced by the polar shoes to very small proportions to avoid this reverse EMF. As the coils outside the pole pieces were doing very little work, a second pair were introduced to partly use up the remainder of the revolution, making four pole dynamos ; and in the larger machines three pairs of magnets and six shoes were added, making eight pole dynamos. The construction of the four and eight pole pole- dynamos necessitates either that there shall be as many pairs of brushes, brush-holders, etc., as there are pole pieces, or the coils which are delivering the same current at the same time must be connected together. Either plan tends to complicate the machine and adds to the cost, especially when it requires repairing ; the presence of a number of cross-connecting wires necessitating the greatest care with the connections, and often very highly skilled labour to manipulate. The next question that arises in connection with the dynamo is, how is the magnetic field, how are the lines of force to be created ? This is done, as already explained, by causing a current of electricity to pass round the coils of copper wire with which the field-magnets are wrapped. But where is the current to come from ? There are two methods of obtaininsf it. First, by driving another dynamo and using its current to Dynamo Machines. 39 excite the field-magnets, as it is termed. This plan mws^ bo adopted with alternators, as the currents which are so often reversed cannot create a permanent magnetic field of much value. It was also used in the early days of electric lighting, when only arc lights were known, to free the magnetic field from the enormous variations in current strength created by the flickering and occasional extinction of the arc. The other plan, which is adopted in all dynamos used for ship lighting, except alternators, is to use the current generated by the armature itself, or a portion of it, to excite the field-magnets, and thereby create the magnetic field. But there are three ways of doing this, and the dynamos in which the different methods are used are termed Series Wound, Shunt Wound, or Compound Shunt Wound. The latter is the one now generally adopted ; it is usually known as the Compound Dynamo. Series Wound Dynamo. In the series wound dynamo the field-magnet coils consist of wire of sufficient thickness to carry the whole current which the armature can furnish with safety, that is, without burning the insulating covering of its coils ; and whatever current strength may be required, or may be used, legitimately or otherwise, outside the dynamo, passes round the field-magnets and creates the magnetic field. This plan, the earliest adopted, presents, as will easily be understood, considerable difficulties in practical work. Should there be, for instance, a break anywhere in the circuit no current can pass, and practically no magnetism is created. In the early days of arc lights, when the carbons of an arc lamp would place themselves just out 40 Electric Lighting for Marine Engineers, of contact, this was by no means an infrequent source of trouble, the engine meanwhile doing its level best to knock itself to pieces, its load having disappeared as if by magic. When the incandescent lamp came in, another source of trouble stood revealed, condemning the series wound dynamo. As will easily be understood, the strength of the magnetic field, the number of lines of force, as electricians mysteriously term it, will depend within certain limits upon the strength of the exciting or magnetising current. So that, using a series wound dynamo, if few lights were on, compared with the possible output of the dynamo, only a small current would pass round the field-magnets, and only a weak magnetic field would be induced, causing the light of the lamps to be low, unless increased speed was given to the dynamo, and vice versd. Further, if what is technically known as a Short Circuit occurred, — that is, if two bare places in the two main leads came into contact, unless the machine was very quickly stopped, — a very powerful current would pass through the coils of the armature and field-magnets, generating sufficient heat to burn the insulation and render the machine unfit for work. Hence about this time the shunt wound dynamo came into favour. The Shunt Wound Dynamo. In the shunt wound dynamo a portion of the current generated by the armature is used to excite the field- magnets. On leaving the brushes, the current divides between the cables, lamps, etc., external to the dynamo, and the wire on the field-magnets ; the latter being of Dynamo Machines. 41 much finer wire than on the series wound dynamo, but making a great many more turns, so that the magnetising power is the same. The magnetising power is measured by the product of the magnetising current and the number of turns it makes round the iron. With the shunt wound dynamo, the EMF at the terminals of the dynamo — that is to say, the number of volts delivered to the cables — varies inversely as the strength of the current that is being used outside. Thus, when the external current is strong, the current passing round the field-magnets is weak, and the EMF the same, and vice versa. The reason that the current round the field-magnets decreases, as the current in the lamps increases, may be given in two ways. When there are two or more paths for a cur- rent, the latter divides itself inversely as the resistance offered by each ; or, in other words, directly as the facili- ties offered, just as if there are two or more pipes to carry off a given quantity of water or steam, the larger pipe will take the most if they are of the same length, and the shorter one, where they are of the same size but different lengths. This explanation, however, is on the supposition that the armature furnishes the same current when many lamps are on as it does when few are on ; whereas, up to a certain limit, turning on more lamps on the plan usually adopted on board ship ; or, as electricians term it, lowering the external resistance, increases the current furnished by the armature ; so that if the cables and lamps take more current in proportion to the field-magnets, as more lamps are turned on, there is more for them to draw from ; and it is not clear from this point of view why the field-magnet wires should get 42 Electric Lighting for Marine Engineers. less current when a number of lamps are burning than when only a few are. The actual cause is the working of Ohm's law in the armature. This formula, it will be remembered, may be rendered : electro-motive force = current x resistance ; and in this form it enables us to measure the charge upon the initial EMF made by any current passing through any conductor, or, more properly speaking, any resistance. Thus, supposing the wires on the armature of a dynamo to have a resistance, when running, of '1 ohm, and the total EMF created by the revolution of the armature at a certain speed, with no current passing in the external circuit, be 50 volts. And let the resistance of the field magnet coils be 50 ohms. When there is no current passing in the outer circuit, the current in the field magnet coils will be all that the armature is called upon volts to furnish. Eoughly, this current will be = -^r^ — :; ^ "^ 50 ohms = 1 ampere, as the EMF at the brushes, where the two ends of the field-magnet coils are connected, will be very nearly that created by the armature. In fact, it will be 5 volts less the product of the resistance of armature X current passing, or50V — •1x1 = 50 — '1 = 49*9 V. But when, say, 50 amperes are passing in the outer circuit, these 50 amperes make a charge upon the 50 volts equal to 50 x •l = 5Vin addition to that made by the field-magnet current. Supposing the total EMF generated by the armature to remain the same, the EMF at the brushes, where the field-magnet coils are connected, will be only 45 V, or thereabouts, and the current passing in the field-magnet coils will now be only C = \% — '9 ampere, or a reduction of 10 %. And this Dynamo Machines, 43 reduction of the magnetising current in the field-magnets coils causes a reduction in the total EMF created by the revolution of the armature, and so on, until a balance is reached. As a consequence of the current in the field- magnet coils decreasing as the current in the outer circuit increases, the EMF at the terminals of the dynamo decreases as the number of lamps in use in- creases, and vice mrsd, so that the behaviour of the shunt dynamo is exactly the reverse of that of the series dynamo. With the shunt dynamo it is necessary to be careful to lower the speed when many lamps are suddenly turned off, as otherwise the EMF increases so much from the above cause, and also from the engine racing, if not very well governed, that the current passing through the other lamps is also very much increased, and they will be seriously strained; the fact before mentioned, that their resistance lowers as the current passing through them increases, adding to the trouble, as owing to this the strain upon the filament increases at a compound ratio. On the other hand, it is possible, by constructing the shunt wound dynamo very large in proportion to the work it has to do, to maintain the current in the field magnets, and therefore the EMF at a given speed, prac- tically constant, whether the machine is running an Open Circuit, as it is termed — that is, with no current in the external circuit, or with the full number of lamps for which the dynamo is constructed. Suppose, for instance, that, instead of the armature resistance in the case quoted above being '1 ohm, it was '01, then the charge for 50 amperes, upon the initial EMF of the armature, would be only '01 x 50 = -5 V, and the difference between the currents passing in the 44 Electric Lighting for Marine Engineers. field-magnet would be as %% to 49*5, which would be very trifling, and it will be seen the difference can be made as small as you choose, by lowering the resistance of the armature. In practice, however, it is cheaper to use a compound wound dynamo, as the increased size of the shunt wound dynamo required to effect approximate self-regulation adds greatly to the cost of the installation. Another point that should be mentioned in connection with the shunt wound dynamo is the, at first sight, curious feature, that when a very large current ought to pasSy or, as electrical engineers term it, when the dynamo is SHORT CIRCUITED, that is to say, when a conductor of very low resistance joins its terminals, no current is furnished, as no EMF is created. As the external resist- ance decreases, the EMF at the brushes decreases ; and though the current in the external circuit increases up to a certain point, after that it also decreases gradually, because the EMF is decreasing faster, owing to the de- creased current in the field-magnets, than the outer resistance decreases, and, consequently, both finally come to zero. Compound Wound Dynamo. In the compound shunt wound, or compound dynamo, the two methods of winding, series and shunt, are com- bined. Part of the magnetism of the field-magnets is induced by fine wire coils connected directly to the brushes or terminals of the machine ; and the other part of the magnetism is furnished by a few thick wire coils through which the current passes, on its way to the lamps. The result is that the machine is always ready to furnish a current, and if properly constructed, that is Dynamo Machines, 45 to say, if the proper proportion between the shunt and series coils is observed, the EMF, or tension, at the terminals of the dynamo, is the same if only one lamp is burning, or even if it be furnishing no current outside the field-magnet wires, or running on open circuit, as it is termed, as when the full number of lamps for which it is constructed are burning. To understand how this perfect regulation is obtained, the action of the two currents may be looked upon as balancing one another. That is to say, as the outside current increases with an increased number of lamps, and the current passing in the shunt coils decreases, so does the current in the series, or main coils, as they are often termed, increase, thereby preserving the magnetism con- stant or thereabouts. But this explanation, though passable for a cursory examination of the question, does not give the whole of the facts, and the reader who wishes to be master of his apparatus will do wisely to pursue the subject a little deeper. To understand the whole action then, look upon the compound machine as a shunt wound dynamo, plus the series coils. It was explained above, that the reason why the current in the shunt coils decreases, as the current passing to the lamps outside increases, is because the passage of the current through the copper wire coils of the armature, makes a charge upon the initial EMF created by the revolution of the armature, within the magnetic field provided for it. It must be borne carefully in mind that EMF is what we have to produce, when we wish to do electrical work. Given EMF, applied to any body, and current follows ; and the one question to be solved, when we have certain 4.6 Electric Lighting for Marine Engineers. work to do, is how to provide sufficient EME to do that work under the conditions. Just as the one ques- tion, after all, in steam-engine work of all kinds, is how to provide such a pressure of steam as will do the work under the conditions ruling. Now, the EME obtainable from any dynamo depends on three things : The speed at which the armature, or bobbin, as it is often called, revolves ; the number of convolutions of wire on the armature ; and, the strength of the magnetic field in which the armature revolves, or, as electricians term it, the number of lines of force passing into its iron core. In any given dynamo the number of convolutions of wire on the armature is, of course, constant ; you cannot take your armature out and put a few more coils on when you require more lights. At least, no ready method has been discovered of doing this. Eurther, with the compound wound dynamo, the speed is also constant. Therefore, if the strength of the magnetic field is to remain constant, while the strength of the current in the inducing shunt coils decreases, the main or series coils must provide as much inducing power, as much magnetism, as many lines of force, as the shunt coils lose. But the main coils have to do more than that even. As the current for the outer circuit, the cables and the lamps, is taken from the ends of the series coils, the EME there must be constant ; and in order that it may be so, the EMF at the brushes and the initial EME created by the revolving armature must be increased to allow for the charge on them, for the passage of the main current through the series coils, so that the latter must be able to provide not only for the loss in the shunt coils but Dynamo Machines, 47 also tor the addition rendered necessary by its own presence. In most compound dynamos a portion of this increase is furnished by the shunt coils themselves, owing to the indirect action of the main coils. Where, as is most common, the ends of the shunt coils are connected to the brushes, as the current passing in the main coil- increases the strength of the magnetic field in whicL the armature revolves, the EMF at the brushes increases, thereby in- creasing the current passing in the shunt coils, and again adding to the strength of the magnetism. This, of course," increases the initial EMF, and so on. So that, in machines constructed on this plan, the EMF at the brushes and the current passing in the shunt coils is higher than would be the case if there were no loss under ordinary conditions as the outer current increases. In some compound wound dynamos, however, the shunt coils are connected to the terminals of the dynamo, so that the main coils do all the work of furnishing the increased magnetism, both to make up for the loss in the shunt coils and for the charge on the initial EMF arising from their own presence. It will be seen that the compound wound dynamo must be far and away superior to either series, shunt, or alternating current dynamos, for ship lighting at any rate; since, providing the engine be properly governed, to run at, or nearly at, one speed, lights may be turned in or out at will, without the fear of damaging any part of the apparatus. Further than this, with the compound machine you may have any light of any power at any place, by merely running your cables to it ; and you may turn the largest light in or out as you please. Moreover, 48 Electric Lighting for Marine Engineers, when you are not requiring lamps in one place, you can use the power of the machine in another without com- plication. It must be remembered, however, that a compound dynamo, constructed to furnish a certain voltage at a certain speed will compound, as it is termed, or will regulate, only at or about that speed. An increase or decrease of speed will upset the balance, though of course it would not be appreciable in the lamps, except to a critical eye, unless the variation in speed was considerable. To understand this, suppose a compound dynamo constructed to run at 600 revolutions, and to furnish 100 volts at its terminals. The voltage at the brushes would, of course, be rather more, call it 102 volts. Now, suppose that from any cause the dynamo is made to run at 700 revolutions instead of 600. The voltage would probably be about 115 volts to 120 volts at the terminals, and 118 volts to 123 volts at the brushes. To simplify matters, suppose that there are 115 volt lamps in use in the ship, and she picks up a 100 volt compound dynamo at some port of call, intending to make it answer for the 115 volt lamps by running faster. Now, the shunt coils being very much more powerful than they should have been to fulfil their proper function, the magnetising power of the shunt coils rising with the speed in a com- pound ratio, there would not be so much for the main coils to make up, when the proper speed for the 115 volt lamps was arrived at. Consequently, when a num- ber of lamps were turned out, the light given by the rest would go down, because the shunt coils would be set Dynamo Machines. 49 necessarily below their proper current in arranging the speed of the dynamo, to avoid overrunning the lamps. In fact, the conditions would be reversed. Instead of the main coils making up the loss of the shunt, the shunt coil would be set to make up, at full load, what the main coils did not provide. The main coils would do their work anyway, so that the shunt coils could only be allowed to furnish enough magnetism to do the rest; and when the armature was deprived of the magnetism of the main coils, or a portion of it, it would not have the strength it should have had, owing to the shunt being below power, unless the speed was raised. The reverse, of course, holds good, if a dynamo is run at a lower speed than it is compounded for. Two other points should be mentioned in connection with compound machines. There will be a limit with every machine, beyond which it cannot be made to compound. It will easily be understood that there is a limit to the strength of magnetism which the iron and other parts of a dynamo will transmit. This limit is well known now by dynamo manufacturers, and they design their machines accord- ingly. But, supposing the current passing in the outer circuit to be increased beyond the limit at which the increased current in the main and shunt coils induces increased magnetism in the iron core of the arma- ture, then the EMF at the terminals must go down, unless the speed is increased, as the charge for the current passing through the main and armature coils has still to be met. In such a case, however, the dynamo would probably be overworked. D 50 Electric Lighting for Marine Engineers. Another point that should be mentioned is, that it is a very difficult thing indeed to compound machines exactly, and therefore it is wise to have attached to the governor of the engine a regulating apparatus, such as a spring whose tension can be adjusted, or a weight whose position can be moved to allow for a slight variation of speed. Only a slight margin is required, and it is a simple matter, if a voltmeter (an instrument that will be described later on) be placed near the engine, to main- tain the light given by the lamps in all parts of the ship quite uniform, no matter how many may be burning. Perhaps the different methods of winding the field- magnets of continuous current dynamos may be under- stood better from the diagrams shown in Figs. 9, 11, and 13, and their behaviour from the characteristic curves shown in Figs. 10, 12, and 14. In Fig. 9 is shown the arrangement of the wires on a series wound dynamo. As will be seen, the whole current, whatever work the machine may be doing, passes round all the coils of the field-magnets. In Fig. 10 is shown what is termed the characteristic curve of the series wound dynamo. That is to say, the history of its behaviour under varying conditions of the external circuit is shown by the form of the curve. In the diagram, horizontal distances represent the strength of the current passing through the machine at the time, and vertical distances the electro-motive force corresponding to each current strength. Thus it will be noticed that the EMF is when the current is 0, and that the former rises very rapidly for a small increase of current up to a certain point, when it remains about stationary, and then slowly bends over. Dynamo Machines, 51 The explanation of this, which has already been given in another form, is, when no current is passing there is practically no magnetism in the field in which the Fig. 9. — Sliomiig, diagrammatically, the winding of Field-Magnets of Series Wound Dynamo. armature revolves, and, therefore, no EMF is generated. As the current increases the iron becomes more and more magnetised, the EMF rising all the time till a point is reached where the additional charge upon the initial 52 Electric Lighting for Marine Engineers. EMF created by the revolution of the armature is greater that the EMF due to the magnetism produced by the additional current when it begins to fall. The curve is useful in many ways. For instance, it is apparent that the only way in which an increased current can be too , / .. z' Sy / -^ \ I 1 1 L $0 00 40 i^ Q 50 4^ 60 go /«'0 Fig. 10. — Characteristic Curve of Series Wound Dynamo. made to pass is by a decreased resistance, and, therefore, that a comparatively small decrease in the resistance external to the machine gives rise to a large increase of EMF. Thus, where incandescent lamps are worked from a series wound dynamo, each lamp that is switched Dynamo Machines. 53 in increases the light given by the others appreciably, without increasing the speed, up to a certain number, while, if many are switched in beyond that number, the light will begin to decrease unless the speed is raised. Fig. 11. — Showing, diagrammatically, the winding of Field-Magnets of Shunt Wound Dynamo. It is understood, of course, that the speed is maintained constant for the curves shown. Another use of this curve is to show at what point comparatively large increases of current produce but 54 Electric Lighting for Ma^'ine E^igineers. small increase of EMF, which is of service in many cases. As an instance of the use of the characteristic curve of the series dynamo, it may be mentioned that series wound machines, which are intended to furnish a number of arc lamps in series, are so constructed that the average VoOs /IC tti^ Vw^ "V N fin \ \ Cn \ \ 4o ] / to ^ ^ ^ ^ jf) ^ 1 O 20 (,0 20 O AMPERES 20 Fig. 14. — Characteristic Curve of Compound Wound Dynamo. times in each second, the smallest number being 40, and the largest in present practice 133. In each period, as it is termed, each complete series of reversals, the EMF passes from zero to a very much greater EMF than that which would correspond with the EMF that would do the same work, if furnished by a continuous current dynamo, thence back to zero. It 6o Electric Lighting for Marine Engineers. then furnishes an EMF in the opposite direction, the terminal that was positive now becoming negative, and vice versa, to the same figure as in the first direction, then returns to zero and so on. As would be shown by the curve, the rise and fall of Figs. 15 and 16. — Showing Siemens' Alternate Current Dynamo, with its Excitor. EMF are both gradual, the form of the curve being that which would be produced by the perpendicular to the horizontal, from various positions of the radius revolving round the centre, forming the curve of sines. The working EMF is, as already explained, that which Dynamo Machines, 6i 62 Electric Lighting for Marine Engineers. is equivalent to the EMF that would generate the same heat in a conductor of a given dimension, if taken from a continuous current dynamo. This working EMF is found to be equal to the square root of the mean of the squares of all the EMF's generated. Fig. 18.— Latest form of Siemens' Dynamo, with Driving Engine. Forms of Dynamos. Fig. 15 shows the only form of alternator that has been used on board ship. It is of the Siemens pattern ; having, as already explained, a double crown of short field-magnets with the starlike armature revolving in the space between them. Dyna77zo Machines, 63 64 Electric Lighting for Marine Engineers, Fig. 16 shows the excitor, as it is termed, that is used with this dynamo. It is a series wound Siemens' dynamo of the early type. Fi?. 20. Fig. 17 shows the early type of Gramme machine, some of which may still be seen doing good work. Dynamo Machines. 65 66 Electric Lighting for Marine Engineers, Fig. 18 shows the latest form of Siemens' dynamo, with its engine attached, on the same bedplate. Fig. 19 shows the Siemens' dynamos that are now employed at central stations on shore. Fig. 22. — Showing Victoria- Brush Continuous Current Dynamo, with Engine attached. Fig. 20 shows the Holmes' dynamo; Fig. 21 is one of Messrs. Scott and Mountain's War Office type Tyne dynamos; and Fig. 22 is the Victoria - Brush with engine. Dynamo Machines, 67 The arrangement of these dynamos are all exactly alike, except those of the two last, the only difference between them being in the form of the field-magnets. Thus in the Crompton, early Siemens, and early Gramme, the field-magnets are of what are known as the double horse- shoe type, or double magnetic circuit type. That is to say, there are in each of these machines two complete liorse-shoe electro-magnets, each having two legs, a yoke piece, which forms part of the frame of the machine, and two sets of wire coils for the electric currents that are to create the magnetism. In the Victoria Brush dynamo, and in the Scott and Mountain dynamo, four sets of magnets are used, with four sets of pole pieces, but only two sets of brushes ; the coils at opposite ends of any diameter of the armature being connected together and so delivering their current to the same brushes. In each of these machines the two horse-shoe magnets are joined by the polar extensions between which the armature revolves. In the Holmes and later Siemens* dynamos, the field-magnets consist of only two limbs, forming a single horse-shoe, having only one yoke and two magnetising coils, the pole pieces forming extensions of the magnet limbs, as in the double magnet type. In the Holmes and Siemens' later dynamos the field- magnets are above the armature, while in the " Norwich " dynamo, another example of the same type, the pole pieces and armature are above the field-magnets. In all but the early Gramme machine the axis of revolu- tion of the armature is at right angles to a plane passing through the centre of the field-magnet limbs and yoke, the brush- gearing being carried by a brass ring, usually working on the outside of one bearing, and arranged to 68 Electric Lighting for Marine Engineers, revolve round the axle, as required to alter the lead of the brushes. In the early Gramme machine the brush-gear is fixed to one of the side frames. Looking after the Dynamo. Starting. — See the bearings oiled, lubricators filled, everything clear of the armature, brushes carefully trimmed and set nicely on the commutator, so that they bear squarely right across its surface, all copper dust and oil carefully wiped off the pole pieces, standards, brush- gear, etc. Throw open all the main switches, then start the engine and run up to the proper speed. When the voltmeter shows that the proper EMF is being furnished, switch on your main circuits, one after the other. The engine and dynamo should never be run without a governor. Trimming and Setting the Brushes. — -Whether these are made of gauze, plate, or wire, always cut them off quite square, and then file a slight bevel on the side that is to bear on the commutator. Set the brushes in their shoes quite square, and see that each brush bears on the commutator for its whole width. See that the brushes bear sufficiently hard on the commutator to make good connection, but not too hard. A guide will be, that pressure which wears both commutator and brush least, and which allows of least sparking. Set the two brushes at exactly opposite ends of a diameter of the commutator. Some commutators have Dynamo Machines, 69 their opposite segments marked. Where this is not done^ they are easily found by counting. In running, look out for the formation of what are termed " flats " on the commutator. The sections of the commutator form sectors of a cylinder, and their surfaces are curved in proportion. They should retain their curve as they wear down. In some commutators, however, the ends of the segments which leave the brush last sometimes become flattened, the surface of the commutator then presenting an irregular curve. When " flats " are formed, sparking is increased at the brushes, and goes on increasing as the irregularity increases. When " flats " are seen to be commencing to form, try to restore the circular form by filing, turning the arma- ture slowly. If the trouble has gone too far to allow of being put right by means of a file, take a cut off the commutator the first opportunity, either in a lathe, or by means of a slide rest fitted to some part of the machine. The latter is by far the best plan, as it avoids the necessity of removing the armature every time. "Flats" generally arise from either badly regulated springs on the brushes, or from some of the segments of the commutator being a little harder than others. If the springs are not strong enough, the brush may jump lightly on leaving each segment, causing more sparking there than should be, with the result that that part of the segment is gradually worn more than the leading part, and so the circular sectional form is lost. A little oil is occasionally of service in reducing both heating and sparking at the commutator, but it should be used very sparingly, — a drop on the tip of the finger, 70 Electric Lighting for Marine Engineers, applied to the surface of the commutator occasionally only. The Lead of the Brushes. — This is of great importance to the sparking and to the wearing of the brushes and commutator. All modern dynamos are made with their two brushes or sets of brushes, positive and negative, attached to a collar that can be revolved round the axis, an insulated handle being provided for the purpose. The proper position for the brushes can always be found by turning the lever handle forward or backward in the direction of rotation, the right position being that at which sparking is least. This position varies with the work the machine is doing, or the number of lamps being taken from it. When only a few lamps are in use, the position of least sparking is not far from a line at right angles to the line joining the centres of the pole pieces. As lamps are turned on, or other work is done by the machine, the position of least sparking is moved farther and farther forward in the direction of rotation. If a dynamo has been standing for some time, it may have temporarily lost its magnetism, and no current can be obtained from it. Striking the pole pieces sharply with a mallet will sometimes remedy this. If not, get a battery of any kind and connect it to the shunt coils at the brushes, the positive pole of the battery being connected to the positive brush, and allow the current to pass for a few minutes, when the dynamo will usually be found to build up. Other causes of failure and how to deal with them will be found in Chap, vil Dynamo Machines, 71 The Use of Accumulatoks on Board Ship and DRIVING OFF THE MaIN ENGINES. The author has seen it gravely suggested that the electric light may be driven from the main engines, and that accumulators may be used to steady the light furnished by the combination. A little thought will show any engineer that both ideas are utterly absurd, from a practical point of view. In the first place, what is required before all things to produce a steady light is a perfectly uniform speed at the dynamo, and as no set of main engines ever can be sure of accomplishing this at all times, it is evident that they are quite unsuitable for the work. Slowing down, for instance, when going into harbour, the lights would go down when ploughing through a heavy sea-way. Again, the lights would go down, with the possible accompaniment of very bright intervals, when the screw was momentarily out of water and the engines racing. But, it is suggested, " all that can be put right by using accumulators ; they will absorb all the variations of the engines." Unfortunately, they will not. You may charge accumulators from a dynamo driven by the main engines, so long as the connection between the dynamo and the accumulator is broken instantly, if the speed of the dynamo falls below a certain figure, but lamps cannot be worked from the dynamo and accumulator combination without a special arrangement which adds to the appara- tus to be looked after. Perhaps the following will explain both these facts: — Each accumulator- cell, when discharging, furnishing 72 Electric Lighting for Marine Engine ef^s. current to the lamps, has an electro-motive force of 2 volts, so that for 60 volt lamps, 30 cells are required; 31 or 32 would usually be employed to allow for the resistance of the cells themselves, and for working down during discharge. While the cells are being charged, and for a very short period after, each cell has an electro-motive force of 2 J volts ; so that, in order that a current may pass from the dynamo through the accumulator, the former must generate an EMF sufficient to overcome this counter EMF, and the resistance of the cells themselves, or about 82 or 83 volts. Now, if the mains leading to the lamps are connected to the dynamo at the same time as the accumulator, the EMF will be very much too high and the lamps will be destroyed, unless a resistance is added to the lamp circuit, to reduce the voltage down to that of the lamps. But immediately you insert a resistance in the lamp circuit, you upset the regulation of your lamps, unless you make your mains larger in proportion, so as to allow for this. Again, if from any cause, the EMF at the terminals of the dynamo falls below that necessary to drive a current through the accumulators, the latter will discharge through the dynamo. So that, unless the charging was very carefully watched, and the accumulators instantly disconnected, when the engines slowed or the belt slipped, it would probably be found, at the end of a long run, that the power had all been wasted, the accumulators having little or no charge in them. Besides the above, accumulators would be sure to give trouble on board ship. However carefully the trays Dynamo Machines. 73 holding the cells were hung, the acid would spill at times when the ship pitched or rolled heavily, and there is quite enough to do on board ship, with the unavoidable presence of salt spray, without adding sulphuric acid. It will be a different matter when, if ever the time comes, the ship itself is driven by a dynamo instead of an engine, with accumulators in place of boilers. For harbour work, too, it is surely better to use steam from the donkey-boiler than to have the trouble of an acx^.umulator. CHAPTER m. CABLES AND BRANCH WIRES. We come now to the consideration of the cables and branch wires used to connect the dynamo, where the current is generated, with the lamps where it is used. The size of these is ruled principally by the same law, Ohm's, that has so often been referred to. Perhaps a better illustration of the working of this law is the behaviour of incandescent lamps themselves as their life increases. Each lamp, whether it be of 8 candle- power, 16 candle-power, 50 candle-power, or upwards, to 1500 candle-power, requires a certain current passing through it, in order to enable it to give out its proper light. When first made, each lamp has a certain resist- ance when burning, which is so proportioned that on the application to its terminals of the EMF for which it is constructed — say 65 volts, 80 volts, etc. — the proper current passes that is required to give out its number of candle-power light. As the lamp burns, however, as those who have used the electric light will have noticed, the bulbs of the incandescent lamps become dark, owing to a deposit of carbon from the filament, the hair-like conductor inside, upon the interior of the glasses. As this carbon is taken from the substance of the filament itself, the sectional area of the latter, its thickness 74 Cables and Branch Wires, 75 decreases as the deposit goes on, and its electrical resist- ance increases in about the same proportion, with the result that less and less current passes through the lamp, causing it to give out feebler and feebler light. All of these cases are ruled by Ohm*s law, and so is every electric circuit and every path by which the current can pass, even though it be not a legitimate path, and one that is doing harm instead of good. As will easily be understood, though there must be a complete path for the current before it will pass, there may be any number of such paths. And, in practice, the distribution of current for electric light on board ship resolves itself into connecting two conductors to the two terminals of the dynamo, and bridging across between these by smaller conductors in which the lamps are included, as shown in Fig. 23. As distribution may have to be effected in different quarters, branch cables are connected to the mains leading from the dynamo, and lamps bridged across these by means of smaller wires again. Fig. 23 shows a pair of mains with two branches, all carrying lamps, and Fig. 24 a larger set, with secondary branches leading out of the first. In practice the conductors for distribution of electric currents for lighting are laid out much in the same manner that steam or gas pipes are. You start with large pipes or mains, to these you join smaller ones, to those again smaller ones, and so on ; the pipes or mains becoming smaller as the current they have to carry becomes less. The difference between electrical distribution and that of gas, steam, or water, is that whereas with the latter the fluid is allowed simply to escape more or less directly, after doing its work, either into the atmosphere, the ground, or some 76 Electric Lighting for Marine Engineers. 5 -o ^^ o- o- -o- ^ Fig. 23.— Diagram of Connections between Dynamo, Cables, and Lamps. 5 I ^^ I ^7^^ o -o- Fig. 24. —Diagram showing Connections between Dynamo, Cables, and Lamps. Cables and Branch Wires, ^ 77 receptacle provided for it ; with the electric current a 'path must lie made for it hack to its generator, or the apparatus it is designed to operate will not work. The size of the cables for distribution is governed by two requirements : first, the heating effect ; and secondly, the fall of EMF due to the passage of the current through the conductor. As already mentioned, the passage of an electric current through any conductor generates heat in the latter in proportion to the square of the current strength x the resistance of the conductor x the time, or H = C^Kt. Thus it will be seen that the higher the resistance of a conductor through which a given current is passing, the greater is the heat generated. In the case of distribution for electric light on board ship, this means that the smaller the conductor the hotter it gets with a given current. The formula also shows that the heating effect increases very rapidly as the current increases, with a given resistance, and for this reason it is better to allow a good margin over and above the size required for the ordinary current strength, so as to provide for an accidental increase of current. Two other points that should be noted in connection with the heating effect are the time factor in the equation, and the condi- tion of the surrounding atmosphere. For regular lighting, for hours together, a larger margin should be allowed in the size of the cables, than where the current is only in them for a short time. So, too, cables in which the full current is always on during lighting hours should be larger in proportion, than those in which a portion of the lights supplied by that cable are turned off for a portion of the time. The case where lamps are burning day and night, of course, requires the most liberal treat- jS Electric Lighting for Marine Engineers, ment on this account. The condition of the surrounding atmosphere, it has been already pointed out, should also be taken into account in the size of cables required for any particular work. The cable leading to the masthead light, for instance, open to the free action of the wind, will safely carry a considerably higher current than a similar cable laid in the stockhold or the engine-room ; and the reason, which is familiar to marine engineers, is the same that was given in connection with the ex- planation of the action of the alternate current machine, viz. the temperature which any body assumes under the influence of a source of heat, is a balance between the heat received from the source and that given off by radiation and convection. In the close atmosphere of the stockhold, or the engine-room, the loss of heat from these causes will be small compared to that in the open air, exposed to every wind, rain, etc., so that the temperature will rise much more in the former place than in the latter, with a given current strength, and therefore a larger margin must be allowed. Of course, it is hardly to be expected that a marine engineer, with all the engines of the ship under his charge, will have time to make an elaborate calcula- tion, taking into consideration all the points enumer- ated, so that some rough and ready rule is required for guidance. The insurance companies' rule is that not more than 1000 amperes to the square inch shall be allowed under all circumstances. It is obvious, however, that this must be capable of considerable modification. Small wires, for instance, have a far larger radiating surface than large Cables and Branch Wires, 79 ones, and can be safely entrusted with larger proportionate currents. Take, for instance, a conductor an inch square, and, therefore, of a square inch sectional area, and compare it with a No. 20 standard wire gauge wire. The D" rod has a surface of 4 inches for radiation ; the No. 2 wire, whose sectional area is toW ^^^ of ^^ D" rod, has a surface of ^ inch, or its radiating surface is only -^ that of the inch rod, and should therefore be allowed to carry 25 amperes instead of only 1. Practice, how- ever, has determined these matters, and, as usual, before theory entered the field, and the table on the following page shows the figures. The figures given represent the currents that the respective wires should safely carry, and the lamps they should feed under the most unfavourable circumstances. In a great many cases larger currents are allowed. For instance, 7 No. 18 wires is often allowed to carry 20 amperes, and 7 No. 16 wires 40 amperes. Cables are made of a number of wires stranded together, as 7 No. 16's, 19 No. 20's, and so on, in order that they may be more flexible than the solid wire of the same sectional area, and because indiarubber and other substances are more easily laid upon a stranded wire, when the cable is of a certain size, than upon a solid wire. In laying cables it is of very great importance to have them as flexible as possible, especially on board ship, where they may have to be bent round sharp corners. The plan of stranding a number of wires together achieves this result in a remarkable degree, but it presents the disadvantage that if salt water does reach the conductor it acts upon the surfaces of the whole of the wires at once, causing 8o Electric Lighting for Marine Engineers. CM H ^ H ^ O 3 m w W ^ fi o ;?; W < o c; ^ Jz; w HH p^ w o H M fW ^ Ph <1 w t-q r/? P s Ph Ph g m ^ r/7 t:^ W k) Ph o ^ yA w <} H 1 The number of lamps in each case is calculated not to allow more than 1000 amperes per square inch to pass through the cable. Cables are made between the sizes given, such as ■^, ^j, ^, ^, and so on, the lamps for which can easily be seen by inspection. •sduiBT: -d-'o 002 !HOA 001 OOOOOOOrHiHi-H(NCOTj- O (N 0» OS ;^ CO i-i rH i-i rH CO urj •sduiBi -d-'o 38 !»0A 001 Oi-I01C0'*OIO1^00O«D00Q00St-I0S I-) r-( rH (M (M O !>. •sdui'BT 'd-'o 91 ^lOA 001 i-(CO»Ot^OOi-li-lrJ0 •sduiBT: -d'-o 001 MOX 08 OOOOr-lfH»Hi-l(N(McooirHt^t^or2- r-i T-\ 1-i CO -^ •sdmtJT; 'd'-o gt ^lOA 09 rH rH CO Tj< lO «0 b- 00 O