276 



ELECTBIC LIGHT. 



cumference sixteen coils, correspondiug to 

 the number of poles in each circular set. The 

 core of each coil is a cleft tube of soft iron, 

 this form having been found peculiarly favor- 

 able to rapid demagnetization. Each core has 

 its magnetism reversed sixteen times in each 

 revolution, by the influence of the sixteen suc- 

 cessive pairs of poles between which it passes; 

 and the same number of currents, in alternate- 

 ly opposite directions, are generated in the 

 coils. The coils can be connected in different 

 ways, according as great electro-motive force 

 or small resistance is required. The positive 

 ends are connected with the axis of the ma- 

 chine, which thus serves as the positive elec- 

 trode ; and a concentric cylinder, well insulated 

 from it, is employed as the negative electrode. 

 This machine is large and cumbrous, being five 

 feet three inches long, four feet four inches 

 wide, and five feet high ; it weighs about two 

 tons. Its illuminating power, when driven at 

 a speed of from 350 to 400 revolutions per 

 minute by a steam-engine, with an expenditure 

 of somewhat over three indicated horse-power, 

 is about that of 2,500 standard sperm candles 

 per hour. 



In 1857 Dr. Werner Siemens, of Berlin, de- 

 vised a form of armature that was a great im- 

 provement on anything previously used, and 

 that has been introduced into 

 several different machines. In- 

 stead of employing coils wound 

 transversely round cores of iron, 

 Siemens, after giving a bar of 

 iron the proper shape, wound 

 his wire longitudinally round it 

 and obtained thereby greatly 

 augmented effects between suit- 

 ably placed magnetic poles. This 

 style of armature is shown in 

 Fig. 10. 



With such an armature Mr. 

 Wilde, of Manchester, construct- 

 ed a machine in 1866 by which 

 he was able to obtain currents 

 of greatly increased intensity. 

 Starting from a small machine, he 

 employed its current to excite an 

 electro-magnet of peculiar shape, 

 between whose poles rotated a 

 Siemens armature ; from this 

 currents were obtained vastly 

 stronger than those generated 

 by the smaller machine. These 

 were conducted round a second 

 electro-magnet of great size, be- 

 tween the poles of which rotated 

 another Siemens armature of 

 corresponding dimensions, and 

 Fio.io. with currents drawn from this 

 effects were obtained, as regards 

 both heat and light, far surpassing anythin* 

 previously known. 



By the substitution of electro-magnets for or- 

 dinary or so-called permanent magnets, another 

 great advance was made in the construction of 



these machines. When an electro-magnet has 

 once been magnetized it permanently retains a 

 small amount of magnetism, and it was discov- 

 ered almost simultaneously by Siemens, Wheat- 

 stone, and Varley, that if a coil be caused to 

 rotate in front of an electro-magnet the residual 

 magnetism will induce a current in the revolving 

 armature. The current thus produced is then 

 used to increase the magnetism of the electro- 

 magnet by being sent through the wire that 

 surrounds it. The strengthened magnet in- 

 stantly reacts upon the coil which feeds it, 

 producing a current of greater strength. This 

 current again passes round the magnet, which 

 immediately brings its increased power to bear 

 upon the coil. And thus there is a continued 

 action and reaction between the magnet and 

 the armature until ultimately very powerful 

 currents are obtained. The machines con- 

 structed on this principle of mutual reenforce- 

 ment are called dynamo-electric machines. 



Numerous machines of this type have since 

 been invented both in this country and abroad, 

 and in some of their various forms they are now 

 generally employed to provide the electricity 

 required for the electric light. The Gramme 

 machine, a French invention, and the Brush 

 machine, invented in this country, may be 

 taken as fair examples of the class, though for 

 effectiveness they rank considerably above the 

 average. The descriptions appended are taken 

 from " Appletons' Cyclopaedia of Applied Me- 

 chanics," where those interested will find the 

 whole subject very fully and clearly treated : 



When a bar magnet is introduced into a coil of in- 

 sulated wire, a temporary current of electricity is set 

 up in the wire, lasting only over the period during 

 which the bar is being introduced. On withdrawing 

 the bar, a secondary current is caused in the wire, 

 which flows in opposite direction to tlie former cur- 

 rent. If the magnet, instead of being inserted and 

 then withdrawn, be carried entirely through the coil, 

 it obviously in its passage comes opposite a succession 

 of spirals or turns of wire. As it does so, it produces 

 in each spiral a current, and these currents will all be 

 in the same direction until the middle point or neu- 

 tral axis of the magnet is reached. After that a cur- 

 rent in reverse direction is caused. Hence, during 

 the passage of the magnet there is produced, first a 

 direct, and then a reversed current. 



fi B' M' K 



FIG. 11. 



If, instead of one bar magnet, two are placed end 

 to end so that the two poles of the same name are 

 in contact, and the coil passed over both, the phe- 

 nomenon last noted will take place in the coil for 

 each magnet separately. If in Fig. 11 the coil .be 

 made to move over these bars, we shall find that in 

 the first quarter of the stroke, as we may term it, 

 from A to M, we shall have a positive current ; in 

 the second quarter, from M to B. a negative current ; 

 again, a negative current from B to M' ; and, finally, 

 a positive current from M' to A . It must also be 

 evident that the same results will be caused if, in- 

 stead of passing the coil over the magnets, the latter 

 were made in circular form, as shown in Fig. 12, 

 and caused to pass through the coil. In order, how- 



