86 KNOWN METHODS OF INCREASING THE TENSION OF CURRENTS. 



once vitiate its indications ; and there is reason to suspect that, under certain circum- 

 stances, even the dilute sulphuric acid with which it is charged may undergo partial de- 

 oxidation, and the evolved hydrogen indicate an amount of electricity less than is actually 

 passing. 



312. We are, therefore, in possession of two distinct methods of indirectly increas- 

 ing the tension of an electric current. The first depends on the reduction of quantity ; 

 the second, on satisfying in the exciting cells affinities which tend to antagonize that 

 which determines the current. 



313. VOLTA'S plan of a reduplicated series unquestionably acts upon the first of these 

 principles. It is a fact admitted on all hands, and, therefore, into the proof it is un- 

 necessary now to go, that the apparent quantity circulating in the whole battery is not 

 greater than that which any one of the pairs could generate. Dr. FARADAY has already 

 shown how an enormous quantity of zinc is thus expended, the equivalent of electricity 

 being entirely sacrificed for the sake of increasing the tension. Let us see what are 

 the facts in the case. The first pair of plates develops by the oxidation of a portion 

 of its zinc a certain quantity of electricity, which, passing through the electrolytic con- 

 ductor, arrives at the second cell; here, however, it is stopped, as a transit without de- 

 composition is impossible, a decomposition which it is unable to effect. Continually 

 tending to pass, without the passage actually taking place, it remains, as it were, on the 

 surface of the second zinc plate, in a condensed state, reacting on the electricity which 

 that plate is generating, compressing and being compressed by it, and, therefore, increas- 

 ing its elastic force. And the same action continually occurs, and increases the ten- 

 sion throughout the series. 



314. A flat spiral coil, or a long connecting wire, obviously acts in the very same 

 way. It opposes a resistance to the passage of the current, and the plate instantly be- 

 comes in a forced state. We might almost regard the electric fluid as existing upon 

 the surface of the zinc, exerting to the utmost its elastic force to pass the barrier, and 

 failing that, compressing the evolved fluid as fast as it is generated, and being compressed 

 by it. This, also, is the case in the pile of Volta. 



315. Thus far, therefore, the riband coil acts simply as a long wire, and this may 

 be regarded as its primary or statical effect. But, besides this, it gives rise to an 

 action of an entirely different character, which Professor HENRY pointed out and ex- 

 plained. In the act of making and breaking contact in a system of which it forms a 

 part, Faradian currents are generated by its successive spirals ; these currents under the 

 latter condition, breaking contact, coincide in direction with the primary current then 

 just ceasing to pass. We must, however, carefully distinguish between these currents 

 and that which induced them. In this respect some philosophers have unguardedly 

 fallen into a very remarkable mistake ; it has been supposed that when a thermo-electric 

 current was passed through this coil, and a spark obtained, the thermal light was seen ! 

 The case is exactly analogous to that in which similar coils pass the jaws of a horse- 

 shoe magnet ; no one supposes that the spark then elicited is due to the electricity of 

 the magnet itself, but is simply a manifestation of the induced current ; the very same 

 thing takes place when the thermal current runs through the spires of a flat coil. So 



