Chap, viii.j THE RHEOCORD. 79 



with the plates a bridge, over which the electricity may 

 pass from the one plate to the other. 



When, however, the slider is pushed along its rail, 

 the current can only get across from the first plate to 

 the second by passing down one wire to reach the 

 bridge, crossing it, and so gaming the second platinum 

 wire, by which it passes back to the second plate. 

 The farther the slider is pushed in the direction of 

 the ivory knife edge, the longer road has the current 

 to travel before it can pass from the first to the second 

 plate, and the greater resistance it encounters on the 

 way. A millimetre scale pasted along the side of the 

 rail indicates the distance between the slider and the 

 brass plates. Now, suppose a current, brought to the 

 binding screw a, has, by passing over the bridge, 

 reached the second plate, it may pass directly across to 

 the third plate, provided a brass plug be inserted 

 between the second and third. If this brass plug be 

 removed, the current is not stopped, for there is 

 attached to the under edge of the second plate a 

 German silver wire (indicated by the dotted line in 

 the figure) which is sunk in the wood, and passes 

 along a considerable way to reach a pulley 1, round 

 which it turns, and goes back to reach the under edge 

 of the third plate, opposite to the second. This wire 

 affords, therefore, a sort of underground pathway 

 connecting the second and third plates, along which 

 the current may travel, when the removal of the 

 plug prevents it passing straight across. But this 

 underground pathway offers much more resistance 

 than the brass plug. It is of the same length as one 

 of the side platinum wires. Similarly between the 

 third and fourth brass plates there is an underground 

 road round the pulley 1', of the same length as that 

 round 1. Between the fourth and fifth plates another 

 German silver wire passes round pulley 2 ; it is twice 

 the length of the first, and therefore offers double the 



