362 Dr. Page on the probable Conduction of 



as perfect insulators, or rather non-conductors of galvanic 

 electricity, might under certain circumstances prove conduct- 

 ors. I took two large sheets of zinc and coiled them toge- 

 ther, the two sheets being separated by a layer of India rub- 

 ber cloth. The sheets of zinc were connected respectively 

 with the poles of a battery, and when the battery consisted of 

 a single pair, no appreciable current passed from plate to 

 plate through the India rubber cloth. When however the 

 battery consisted of twenty compound pairs, a slight current 

 passed, as indicated by a delicate galvanoscope. I made no 

 further investigation or application of this fact till 184-3, when 

 recurrence was had to this feature to solve some difficulties 

 experienced in the projection of a line of Morse's telegraph 

 between this place and Baltimore. Ten miles of lead pipe 

 'containing four well-insulated wires had been laid in the 

 ground, and upon trying these wires respectively, making the 

 wire one-half of the circuit, and the lead pipe the other half, 

 with a battery of intensity, a current could be established 

 through any one of the three extra wires. Being consulted 

 by Prof. Morse as to the probable cause of this cross Jiring, as 

 it was technically called, the solution seemed to me to be ob- 

 vious in view of the above experiment. The reasoning was 

 sufficiently plausible to induce Prof. Morse to abandon the 

 undertaking of the pipe, and resort to his original plan of 

 raising the wire upon posts. The explanation was simply 

 this, viz. that the insulating or non-conducting material would 

 under any circumstances conduct the current ; but that in some 

 cases the amount transmitted could not be appreciated by 

 any known test however delicate, was a postulate subsequently 

 borne out. Does not such a conclusion follow directly from 

 the law of Pouillet, that the conducting power of wires is 

 directly as their cross section and some inverse ratio of their 

 length, in connexion with the well-established laws of the dif- 

 ferent conducting powers of metals? For example, copper 

 having from four to six times the conducting power of iron, a 

 wire of iron, to equal in this respect a wire of copper, should 

 be of from four to six times the size. Let this rule be applied 

 to poorer conductors, and we may infer that the poorest con- 

 ductor, or what has been usually considered a non-conductor, 

 would become a conductor, if the area of its cross section 

 were indefinitely increased and its length remained nearly 

 nothing. In the case of the wires in the lead pipe — the one 

 for instance joining the two poles of the battery — is separated 

 from that lying next to it and the lead pipe, by a layer of thin 

 non-conducting material throughout its length ; and if we 

 suppose the width of this layer in contact to be ^th of an inch, 



