concerned in the Phanomena of ordinary Electricity, §c. 211 



and on this place a magnetic needle within an eighth of an inch 

 of the wire. The apparatus, set standing in a proper vessel, is 

 to be so turned that the copper horizontal wire shall coincide 

 with the direction of the magnetic needle. Pour a sufficiency 

 of very dilute sulphuric acid into the vessel, and instantly the 

 needle will be deflected between 30° and 40°. This is the same 

 degree of deflection which the galvanometer needle suffered in 

 Colladon's experiment with common electricity \ but in order to 

 obtain that amount, he was obliged to use a galvanometer coil 

 consisting of 500 turns ; that is, he was under the necessity of 

 employing the combined effect of 500 wires carrying an intense 

 power of common electricity; whereas in the experiment just 

 described, where the effect was truly voltaic, the same amount of 

 deflection was produced by a single wire carrying the most feeble 

 voltaic electricity. 



The experiments of M. Colladon were repeated and varied by 

 Professor Faraday. He employed a Leyden battery having a 

 surface of 3510 square inches of coated glass. By successive 

 discharges of this battery through the galvanometer, conducted 

 by a wet thread 4 feet long, the needle at length suffered deflec- 

 tion to the amount of 40° on each side of the line of rest. He 

 obtained the same deflection also by electricity direct from the 

 prime conductor, without the battery*. 



The plate of the machine used by Faraday, as described by 

 him, is 50 inches in diameter; it is furnished with two sets of 

 rubbers ; one revolution of the plate will give from ten to twelve 

 sparks from the conductor, each 1 inch long. The battery which 

 produced deflection on the needle had been charged with forty 

 revolutions, consisting of 440 one-inch sparks ; and as it was 

 found necessary to repeat the discharge several times in order to 

 produce a deflection of 40°, at least 2000 one-inch sparks were 

 required for that purpose. 



These deflections, however, both in the case of the battery and 

 of the conductor, derived assistance from other sources beside 

 electricity, which greatly magnified their amount. The swings 

 of the needle were promoted, from very small arcs, to one of 40° 

 by alternate circulation and interruption of the electric current. 

 lb in-,' the amount of angular deviation was rather a semblance 

 than the reality of voltaic deflection. A heavy pendulum may 

 he made to oscillate in considerable arcs by causing the weakest 

 tore.' to act on it at interval! corresponding with the time of its 

 oscillations. Since the deflection, assisted as it was by the me- 

 thod of productionj was but 10", how feeble must have been the 

 force that produced it ! 



Mr. Armstrong, with an hydro-electric machine which dis- 



* Researches, [>. 85. 



P2 



