Electrical Machines. 253 



Art. V. 



>/ 



rfficacy of Elect 



by R. Hare, JVL D., Professor of Chemistry in the University of 

 Pennsylvania. Communicated by the author. 



Sometime since, in looking over a volume of Cavallo's Electricity, 

 I was surprised to observe that in order to give the greater efficacy 

 to an electric machine, he advises that the cushion, or negative pole, 

 should be made to communicate advantageously with the earth. As 

 the means of accomplishing this object he suggests a conducting com- 

 munication " with moist ground, with a piece of water, or with the 

 iron work of the water pump." 



Jt appears from the following passage in Turner's Chemistry, a 

 work generally of great merit, that the erroneous impression which 

 gave rise to these suggestions, has been adopted by a more modern 

 author. We find, oaee 77, American edition, the following allega- 



tion. 



" The electricity which is so freely and unceasingly evolved during the action of 

 a good electrical machine, is derived from the great reservoir of electricity, the 

 earth. This is obvious from the fact that if the whole apparatus is insulated, the 

 evolution of electricity immediately ceases ; but the supply is as instantly restored, 

 when the requisite communication is made with the ground. In the state of com* 

 plete insulation, the glass and prime conductor are positive as usual, and the rub- 

 ber is negatively excited ; but as the electricity then developed is derived solely 

 from the machine itself, its quantity is exceedingly small. When the machine is 

 used, therefore, the rubber is made to communicate with the earth. As soon as 

 friction is begun, the glass becomes positive and the rubber negative ; but as the 

 latter communicates with the ground, it instantly recovers the electricity which it 

 had lost, and thus continues to supply the glass with an uninterrupted current. If 

 the rubber is insulated, and the prime conductor communicates with the ground, 

 the electricity of the former, and of all conductors connected with it, is carried away 

 into the earth, and they are negatively electrified." 



I conceive that the earth has never, of necessity, any association 

 with the phenomena of the electric machine ; of which the power is 

 evidently dependent on the efficacy of the electric, in transferring the 

 fluid from the negative to the positive conductor. When the conduc- 

 tors are both insulated, by the revolution of the electric they are brought 

 into states of excitement as opposite, as the power of the machine is at 

 the time competent to produce. If, under these circumstances, with 

 one end of a metallic rod, (terminating in a metallic ball, or other suita- 

 ble enlargement, and held by means of an insulating handle,) we toucb 



Vol. XXIV.— No. 2. 33 



