94 Prof. J. A. Fleming on the 



resistance being introduced. The larger carbon also could 

 be rendered incandescent by the proper electromotive force. 

 If the smaller carbon was kept cold and employed simply as 

 a third electrode or middle plate, all the phenomena previously 

 described as happening with metal middle plates of aluminium 

 or platinum took place. If the small (cold) carbon loop is 

 connected through a galvanometer with the positive electrode 

 of the larger carbon loop when this last is rendered incan- 

 descent by a current, we find as usual a current of a few 

 milliamperes passing through the galvanometer from the 

 positive electrode ot the larger carbon to the small carbon* 

 If the small carbon (still cold) is connected through the 

 galvanometer to the negative electrode of the larger carbon 

 we get no current. This is the normal effect, and it is the 

 same for a cold carbon conductor used as a middle plate as 

 for a metal middle plate. 



Experiment 22. — The next experiment consisted in making 

 this small carbon incandescent by an insulated secondary 

 battery, appropriate resistance being inserted so that the 

 carbon was brought to the normal condition of temperature as 

 indicated by its incandescence. When this was done the 

 galvanometer was inserted between the positive electrode of 

 the large carbon loop and one of the electrodes of the small 

 carbon loop. A current was obtained as before. On con- 

 necting the galvanometer between the negative electrode of 

 the large carbon loop and one of the electrodes of the small 

 carbon loop a current of nearly equal value was now obtained. 

 In this last experiment it was found to be immaterial whether 

 the terminal of the galvanometer was joined to the positive or 

 to the negative electrode of the small carbon loop. Hence 

 we find that when the small carbon loop is not incandescent 

 and is used as a middle plate or electrode, it is brought down 

 together with the insulated battery attached to it to the same 

 potential as the negative end of the large incandescent carbon, 

 and we get as usual a current through a galvanometer con- 

 nected between the positive electrode of the large incandescent 

 carbon and any point on the small cold carbon, and no current 

 between the negative electrode of the large hot carbon and 

 the small cold one. On rendering the smaller carbon loop 

 incandescent this is all changed. The smaller carbon, now 

 hot, is not brought down to the potential of the negative ends 

 of the larger carbon, and we get a current through a galvano- 

 meter connected between either positive or negative electrode 

 of the large hot carbon and any point on the circuit of the 

 smaller equally hot carbon. 



§ 25. Experiment 23. — With this same vacuum tube having 



