120 Prof. J. C. Poggendorff on the Induction Apparatus 



means of a liquid or solid insulator. In this peculiarity A'^olta's 

 induction apparatus differs essentially from the magneto-electric 

 machine. 



First case. — When the poles of the induction coil are connected 

 by a metallic wire or by a good conducting liquid, the induction 

 current consists of two parts, or two alternate currents, one in 

 the opposite, and the other in the same direction as the inducing 

 current, corresponding respectively to the moments when the 

 circuit of the latter current is closed and broken. 



This can be detected by the galvanometer. When the induc- 

 tion wire is very thin and long, and a galvanometer is introduced, 

 no deviation is observed ; but when the said wire is thicker and 

 shorter, or the inducing current stronger, the phrenomenon of a 

 twofold deviation, which I described in 1838, is observable*. This 

 is caused, of course, by quick, successive magnetizations of the 

 needle in opposite directions. 



If water or dilute sulphuric acid is introduced into the cur- 

 rent between platinum plates, oxygen and hydrogen are liberated 

 at each plate, and after the cessation of the current no trace of a 

 polarization of the plates can be detected. 



In a similar manner, moistened iodide of potassium paper, 

 when put in contact with the poles, shows a separation of iodide 

 of equal sti'eugth at each pole. 



An electric thermometer rises, the direction of the current 

 being indifferent to it, but a thermo-electric current receives no 

 charge. 



Lastly, the condenser has no influence upon these phsenomena. 



Second case. — When the poles are separated by a stratum of 

 air or gas, even although the stratum be very thin, only one of 

 the two induction currents acts, i. e. the one produced by break- 

 ing the galvanic current; the other, produced by closing the 

 current, remains without external action in the coil, which on 

 that account obtains fixed poles. 



This is evident in the phsenomena of tension, which are exhi- 

 bited when the induction coil is unconnected, that is to say, when 

 its poles are separated by a great thickness of air. If during its 

 activity an electrometer be brought for a moment into contact 

 with one pole of the apparatus, it will be quite accidental whether 

 it will become charged positively or negatively, as the two elec- 

 tricities change places continually on each pole. If the electro- 

 meter be placed near enough to allow sparks to spring over, only 

 one kind of electricity is obtained from each pole, i. e. that which 

 belongs to the pole when the current is interrupted. 



This is still better shown when one of the poles is connected 

 with the earth by means of a wire. The electrometer, when held 

 * PoggendorfFs Annalen, vol. xlv. p. 353. 



