THE POWER OF CONTACT 



By CHARLES E. BEN HAM. 



Over a hundred years ago a memorable discovery 

 was announced by Volta, who claimed to have 

 proved that by the mere contact of a disc of copper 

 with a similar disc of zinc the two metals became 

 electrified, the copper negatively, the zinc positively. 

 Much dispute arose over the experiment : for though 

 Volta repeated it hundreds of times, always with 

 just the same result, many men of science would not 

 believe in it. It seemed to oppose the fundamental 

 doctrine of the conservation of energy and to suggest 

 the heresy of perpetual motion ; and so the " contact 

 theory " of electricity was rejected 

 on the principle that " it can't be; 

 therefore it isn't." It seems strange 

 that instead of arguing as to whether (f\ 



it was possible or not, the scientific 

 world did not try the experiment 

 for itself. It was an exceedingly 

 simple one, one that anybody can 

 try with a condensing electroscope, 

 and one that never fails to verify 

 exactly what Volta announced. But 

 instead of putting the matter to a 

 practical test, the learned world, 

 reeling quite convinced that there 

 must be a mistake somewhere, con- 

 tented itself with abandoning the Figure 150 

 contact theory as impossible, and for 



gives up to the vessel that receives it a constantly 

 accumulating charge of electricity. This is but one 

 special case of a widely extending law, and it seems 

 probable that any two dissimilar substances when 

 brought into contact and separated, are charged 

 with opposite electricities. What is called 

 frictional electricity is another particular case of 

 the same law. When a piece of sealing wax is 

 electrified by rubbing it with flannel it is not the 

 force of friction that is converted into electrical 

 energy. The rubbing is only the means by which 

 two such non-conducting bodies 

 are brought sufficiently into contact 

 to produce the effect. It is for 

 this reason that frictional electric 

 machines involve such a wasteful 

 expenditure of force. An over- 

 whelming proportion of the energy- 

 used is merely wasted so far as 

 the direct production of electricity 

 is concerned. The electrical output 

 is only equivalent in energy to the 

 amount of force required to bring 

 glass and rubber into contact and 

 to separate them again. All the 



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a long time it remained universally 

 discredited in spite of the fact that anybody could have 

 easily demonstrated its truth at any time. At last 

 came Lord Kelvin, who took the wiser course of 

 repeating Volta's experiment. He did it in a new 

 and ingenious way, which avoided all possibility of 

 any other cause than mere contact coming into action, 

 and to his own astonishment, and that of the whole 

 scientific world, he found that Volta was perfectly 

 right. The discovery led him on to make other 

 tests, all of which com- 

 pletely confirmed the 

 mysterious and inexplic- 

 able fact that when one 

 metal touches another, 

 a separation of elec- 

 tricities occurs, one 

 becoming positively 

 charged, the other 

 negatively. Copper 

 filings passing through 

 a zinc funnel are each 

 in turn brought into 

 contact with the zinc, 

 and the falling stream 



FlGUKK 151. 



rest of the rubbing is labour spent 

 in overcoming resistance to such 

 intimate contact. The modern static 

 machines, such as the Wimshurst, 

 require no friction, but depend upon a different 

 principle, the principle of multiplying an infinitesimal 

 original charge by the process of induction. That 

 original charge is derived from contact, either 

 contact between a small metal brush and the tin 

 sector on the glass disc, or it may be by the mere 

 contact of that sector with air. 



According to Volta's principle the electricity of 

 a battery of zinc and copper plates in acid water is 



due not to the chemical 

 action of the acid on 

 the plates but to their 

 contact with each other, 

 the acid water merely 

 acting as a conductor 

 to enable the current 

 to pass. The chemical 

 action is to be looked 

 upon rather as the effect 

 than as the cause of the 

 electricity — an unfor- 

 tunate effect, indeed, 

 from one point of 

 view, because the acid 







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