370 Mr. 0. J. Lodge on a Mechanical Illustration 



exerting a constant electromotive force. But we also, in 

 the consideration of electrolysis, saw a little further into the 

 real action going on in a voltaic cell ; and we shall probably, 

 in a subsequent communication, try to look into the internal 

 mechanism of a thermoelectric joint. 



There are other ways, however, of moving the cord than by 

 a weight. We may attach a piece of elastic to it, stretch the 

 elastic, and fasten its free end to a nail ; the elastic would then 

 tend to unstretch itself ; and it would pull the cord a certain 

 distance, quickly or slowly according to the resistance of the 

 circuit. This represents a current due to the subsidence of a 

 given difference of potential such as would be produced in a 

 wire by arranging it between a sphere charged with a certain 

 quantity of electricity and the earth, or by using it to join up 

 the coatings of a charged Leyden jar. 



A third way of moving the cord would be to wind it round 

 an axle and then to turn the axle with a winch. This would 

 represent electromotive force generating difference of potential 

 or doing other work at a limited rate, as in a magnetoelectric 

 or an ordinary electric machine. To represent the latter, the 

 axle must be thin and the handle turned slowly ; the current 

 produced would then be very weak at the best of times, but 

 it would be little affected by increasing the external resistance, 

 and it would be able to break through the elastics of a di- 

 electric, i. e. to produce a disruptive discharge. If slipping 

 takes place between the cord and the driving-apparatus, it 

 means that energy is being wasted by some means, such as 

 friction in a machine or " local action " in a battery. 



Effects of rapidly reversed small Electromotive Forces. 



§ 18. Instead of pulling the cord continuously in one direc- 

 tion or the other, let us shake it rapidly to and fro. In a per- 

 fect dielectric no energy will be lost ; for the buttons are car- 

 ried with the cord, and their elastics exert a restoring force, 

 which tends of itself to make the cord oscillate with a succes- 

 sion of simple harmonic motions. But in a metallic conductor 

 the cord simply slips through the buttons, no restoring force 

 is called out, and the energy of the displacement is lost by 

 friction. 



These small to-and-fro motions represent the effect of waves 

 of light falling on the body ; for, whatever the cord does or 

 does not correspond to in nature, it certainly represents a con- 

 tinuous incompressible medium connecting together the par- 

 ticles of bodies ; and supposing light to be small and rapid 

 oscillations of such a medium, it would be represented by small 

 and rapid oscillations of the cord. But the linear extension of 



