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



electricity : it', on the other hand, we can abstract some of the 

 cord from the body, it is negatively charged. The capacity 

 of the body will be measured in the ordinary way by the excess 

 of cord which is got into or out of the body, divided by the 

 difference of potential required to get it in or out. 



Model to illustrate the properties of Dielectrics. 



§ 2. Now let us apply these considerations to the construction 

 of a model which shall represent the charging of a Leyclen jar. 

 In fig. 1 (Plate III.) ABCD represents the cord, passing over 

 four pulleys, A, B, C, D,and through eight buttons, 1, 2, 3, &c, 

 which represent eight strata in the dielectric of a Ley den jar 

 or other accumulator ; these buttons are supported on elastic 

 strings, and they grip the cord tightly so as only to allow it to 

 slip through them very slowly and with considerable resist- 

 ance, if at all. Continuous electromotive force is represented 

 as being applied to the cord at D, in the shape of a weight W, 

 tending to turn one of the pulleys on which the cord runs ; 

 and there is a contact-breaker at C, represented by a screw S, 

 which is able to clamp the cord, or in other words to make 

 the resistance of the external circuit infinite. The inner coat- 

 ing of the jar may be represented by the space between A and 

 the first button ; the outer coating by the space between the 

 last button and B. The part ABCD of the circuit, since it 

 contains ordinarily next to no resistance, represents very good 

 metallic conduction. 



Suppose initially that the cord is free at C, and that the 

 weight W has never been applied ; all the elastics will be ver- 

 tical, the buttons will be at the same distance from one another, 

 the distance A 1 equal to the distance B 8, and there will be no 

 excess of tension in any part of the cord. This represents an 

 uncharged Ley den jar, its inner and outer coating connected by 

 a metallic wire. 



Introduce into the circuit at D an electromotor (like W). 

 The potential towards A rises, the potential towards C falls, 

 and the cord begins to move from A to B. The first effect of 

 this motion is to displace the buttons all the same distance to 

 the right — to such a distance, namely, that the sum of the ten- 

 sions in their elastics is equal to the weight W. Then the 

 cord stops. The dielectric is polarized; and its condition is 

 shown in I, fig. 2. Electricity has been displaced in it in the 

 direction of the electromotive force: the inner coating Al 

 has gained cord; it is charged positively; the outer coating 

 B 8 is charged negatively ; pressure is exerted on the left face 

 of every button, and tension on its right face ; that is, every 

 stratum is electrified positively on the side from which the 



