344; Dr. Draper on the Use of a Secondary Wire as a 



state, reacting on the electricity vvliich that plate is generating, 

 compressing and being compressed by it, and therefore in- 

 creasing its elastic force. And the same action continually 

 occurs, and increases the tension throughout the series. 



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 becomes 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. 



Thus far therefore the ribbon coil acts simply as a long 

 wire, and this may be regarded as its primary or statical et- 

 fect. But besides this, it gives rise to an action of an entirely 

 different character, which Prof. Henry pointed out and ex- 

 plained. In the act of making and breaking contact in a sy- 

 stem of which it forms a part, Faradian currents are generated 

 by its successive spirals ; these currents under the latter con- 

 dition, breaking contact, coincide in direction with the pri- 

 mary 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 un- 

 guardedly 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 si- 

 milar coils pass the jaws of a horseshoe magnet; no one sup- 

 poses 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 far as I am 

 informed, the magnetic spark and the true thermo-electric 

 spark have never yet been seen. 



These observations are made, in order that I may not be 

 misunderstood. It is not my object to consider the different 

 arrangements that can generate a Faradian current, and there- 

 fore in this point of view I dismiss the flat spiral. 



We now come to the fourth and last proposition, which is, 

 " That the law which regulates the connexion of the diminu- 

 tion of quantity or condensation, with the increase of tension, 

 is the same as that which regulates the analogous phaenomena 

 of ponderable elastic fluids." 



I have not hesitated to use the terms ' compression,' ' con- 

 densation,' ' elastic force,' in reference to electricity, though 



well know such an application is unusual. But it has 



