I70 



NATURE 



[July I, 1875 



MA GNE TO-ELECTRIC MA CHINES * 

 III. 

 TJ^ROM this property of the Gramme machine it may be 

 ^ employed to measure by the method of opposing cur- 

 rents any electromotive force. For this purpose it is only 

 necessary to ascertain the velocity of rotation of the ring: 

 when the equilibrium between the currents is established. 

 This may be measured in one of two ways — by the velo- 

 cimeter of Deschiens, or by a chromoscopic diapason. 

 The mode of operating with the latter when applied to 



Fig. 7. — Gramme machine for metallic precipitations. 



the Gramme machine is thus described in M. Breguet's 

 work. On the axis of the ring is mounted a small plate 

 whose plane surface is covered with lamp-black by hold- 

 ing it over a candle. A tuning-fork vibrating one hundred 

 times in a second, and carrying at one end a little style, 

 is held in the hand, or, still better, fixed on a special 

 support. At the precise moment that the two electro- 

 motive forces are shown by the galvanometer to be equal, 



*.,'^'*«^?"l>stance of a Lecture, with additions, -delivered at the' Belfast 

 i-hilosophical Society, March 17, by Dr. Andrews, F.R.S., L. & E, (Con- 

 tinued from p. 132.) - ' ^ 



the style is brought into contact with the blackened sur- 

 face of the plate, upon which it traces a sinuous line. A 

 very short contact is sufficient to give the required result. 

 On stoipping the machine, it will be seen to what fraction 

 of the circumference ten sinuosities of the line traced on 

 the plate correspond, from which it may be inferred in 

 how many hundredths of a second the entire revolu- 

 tion of the ring has been accomplished. It is stated 

 that if the ring in the Gramme machine be turned 

 at a perfectly steady rate, the current produced will 

 be more rigorously constant even than that of a 

 Daniell's battery in good working order. 

 Fig. 7 represents a machine constructed 

 with electro-magnets in 1872 by M. 

 Gramme, which, with six others of the 

 same kind, is in use in the well-known 

 galvanoplastic establishment of Chris- 

 tofle and Co., of Paris. These machines 

 weigh 750 kilogrammes, and the weight 

 of copper used in their construction is 

 about 175 kilogrammes. With a small 

 engine of one-horse power, one of them 

 will deposit 600 grammes of silver per 

 hour. By some recent modifications in 

 its construction this machine has been 

 improved so as to increase the weight 

 of silver deposited per hour to 2,100 

 grammes, or above 4i lbs. In Figs. 8 

 and 9 we have the forms of the Gramme 

 Machine now in use for the production 

 of the electric light. They are improve- 

 ments on the machine which was tried 

 on the Clock Tower of Westminster 

 Palace. This machine had the defect 

 of becoming heated while at work, and 

 of giving sparks between the metallic 

 bundles of copper wire and the conduc- 

 tors from the helices. In the machine 

 represented in Fig. 8 these defects are 

 said to have been completely remedied. 

 The entire machine weighs 700 kilo- 

 grammes, and there are 180 kilogrammes 

 of copper in the electro-magnets, and 

 forty kilogrammes in the two rings. It 

 produces a normal light of 500 Carcel 

 burners; but, by augmentiiig the velo- 

 city, it is asserted that the amount of 

 light may be doubled. It does not be- 

 come heated, nor does it produce any 

 spark where the brushes are applied. 



In Fig. 9 we have the latest improve- 

 ments devised by M. Gramme for pro- 

 ducing the electric light. In this ma- 

 chine there are only two bar electro- 

 magnets and a single moveable ring 

 placed between the electro -magnets. 

 Its weight is 183 kilogrammes, and the 

 entire weight of copper used in its con- 

 struction, both for the ring and for the 

 electro-magnets, amounts to forty-seven 

 kilogrammes. Its normal power is 

 about 200 Carcel burners, but this can 

 be greatly augmented by increasing the 

 velocity. It may be interesting to give the results of some 

 experiments with this machine. 



