418 Prof. Tyudall on Vibrations produced by an Electric Current. 



current had been interrupted — doubtless in consequence of the 

 heat which the current had excited. 



The manner in which I conceive the current to act will be 

 best understood if we suppose a flat rocker to rest upon two 

 pointed wires, coming alternately into contact with them as it 

 oscillates. Starting the rocker by the hand, let the current be 

 established ; it passes between the rocker and the wire on which 

 it chances to rest at the moment the circuit is made ; encounters 

 resistance, heats the wire, and expands it; it also produces a 

 nipj)le on -the rocker itself, which is tilted up in consequence, 

 and comes into contact with the other point. The current 

 passes through the new channel thus opened to it, heats and 

 expands as before, the rocker being again tilted. Every time 

 the rocker comes into contact with the wire, the heating agent 

 is there, ready to lift it ; and thus the vibrations are rendered 

 permanent. 



^Vith regard to the method of cooling adopted by Professor 

 Forbes, I should not conclude that it would suspend the pha;no- 

 menon. The interposition of water does not materially alter the 

 resistance, and cannot abstract the heat with sufficient speed to 

 prevent the expansion. Everybody knows that the electric light 

 can be produced under water. The experiment with the carbon 

 is no more than I should expect ; for not only is the carbon lifted 

 by its own expansion, due to the intense heat developed in this 

 substance by electricity, but the pores of the substance are also 

 charged with air and gases, which must expand under the cir- 

 cumstances with almost explosive force. But I think the experi- 

 ment with the bismuth, where Prof. Forbes finds that a current 

 fifteen times the strength of that used with the carbon is unable 

 to produce vibrations, presents a formidable obstacle to the hy- 

 pothesis that the efi"ect is due to any repulsive power inherent 

 in the current itself. Prof. Forbes refers this anomalous result 

 to a " quelling power " possessed by the bismuth. The quelling 

 power consists, I imagine, in the fact that bismuth is both a 

 friable substance and has a very low melting-point. The heat 

 that produces nipples on more refractory metals is sufficient to 

 fuse it ; and hence its inability to produce the effect. Never- 

 theless, by properly arranging the strength of the current, it 

 is quite easy to obtain vibrations with bismuth also. 



In support of the above opinion, I would say that energetic 

 vibrations are obtained with the refractory metals, and the reverse 

 with the fusible ones. The alloy which is technically called 

 fusible metal, exercises a quelling power far superior to bismuth ; 

 and the want of localization of the heat at the surface, so de- 

 trimental to the success of the Trevelyan experiment, is apparent 

 here also in the deportment of silver. Other circumstances 



