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



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p l3 p 3) &c. [the current due to conduction through each 

 stratum]. The rate at which the cord slips through each 

 button. 



/i>/a> ^ c> - [ lne ele^ric displacement]. The distance of each 

 button * from its mean position. 



?/ 1? i< 2 , &c. [the total current through each stratum, due 

 partly to conduction and partly to variation of displace- 

 ment] . The rate at which the cord passes by a point in 

 each stratum fixed in space. 



7\, r 2 , &c. [the specific resistance referred to unit of volume]. 

 The coefficient of " friction v (see § 10) between the cord 

 and each button. 



Kij fc 2 , &c. [the coefficient of electric elasticity, which is 



equal to ^ where K 1? K 2 , &c. are the specific inductive 



capacities of the several strata (see Maxwell, art. 60)]. 

 The " coefficient of elasticity " of each elastic, or the force 

 tending to replace a button when displaced unit distance 

 from its mean position (see § 10). 



E [the electromotive force due to a voltaic battery placed 

 in an}^ part of the external circuit] . The force applied at 

 an external point such as D (fig. 1) to move the cord, 

 or the tension in the part of the cord between D and C 

 minus that in the part between D and A. 



u [the current in the external circuit and battery]. The 

 rate at which the cord passes over the fixed pulleys. 



Q the total quantity of j elec |ncity j ^^ hag pasged 



through the external circuit up to the time t Q = I udt. 



E [the resistance of the battery with connecting wires]. 

 The drag on the cord in the external circuit, including elec- 

 tromotor at D, pulleys, and any other rubbing surfaces 

 as S. 



* In the model, then, electric displacement in a dielectric is accompa- 

 nied by displacement of matter. It may possibly be so also in fact j and 

 if so, on suddenly charging or discharging a condenser, a sudden kick 

 might be observed, the body as a whole being thrown forward with a mo- 

 mentum equal and opposite to that of its displaced particles. But this is 

 not at all necessary, because one does not see to what the other ends of the 

 elastics can be attached except to another set of buttons which get dis- 

 placed in the opposite direction by a negative current simultaneous with, 

 and equal to, the recognized positive current. (This set of buttons must 

 be understood to take the place of the fixed beams employed in the 

 model for convenience.) As, however, there is no reason apparent why the 

 masses of the two sets should be equal, a kick might be observed equal 

 to the difference of the two momenta; at any rate it is worth looking for. 



