imitates the Behaviour of Dielectrics. 231 



in diameter, and each succeeding piston has a greater area 

 cut away, the top piston having just sufficient metal left to 

 make the spring come to rest without vibration after being 

 compressed. The cylinder is filled with a mixture of machine- 

 oil and vaseline. To the top piston is attached a brass tube 

 to which is fastened a compound crossbar B. The latter 

 is built up of two pieces of soft iron separated by a piece of 

 sheet brass. The upper part of the crossbar forms the 

 armature of an electromagnet M, the yoke of which has a 

 hole drilled in it to form a guide for the rod R. Two 

 electromagnets M L and M 2 carrying weights W x and W 2 can 

 be attached to the lower plate of the crossbar, and can be 

 instantaneously detached by breaking the circuit. The upper 

 magnet M, by its attraction of the upper plate of the crossbar, 

 keeps the top piston in its zero position. The magnets Mi 

 and M 2 being attached, on breaking the circuit of M the 

 crossbar moves downward under the action of the weights, 

 and this corresponds to the charging of the condenser. If 

 the circuit of M x and M 2 is now broken, the crossbar moves 

 under the action of the springs alone, and its motion corre- 

 sponds to the discharge of the condenser. In order to draw 

 diagrams of the motion, a pencil which runs in a guide is 

 attached to the crossbar. This pencil moves over the surface 

 of a clockwork-driven recording-drum which runs in bearings 

 attached to the back of the teak framework of the model. 



4. A typical diagram drawn by the model is shown in 

 PI. V., fig. I ; the ordinates represent displacement of the 

 crossbar, and abscissas represent time in arbitrary units. On 

 applying the weight to the crossbar the latter first moves 

 down very quickly, as shown by the steep initial portion of 

 the curve in the diagram. This is followed by the more 

 gradual compression of the lower springs, the crossbar moving 

 with gradually decreasing velocity. At the point B on the 

 curve the weights were detached from the crossbar, and the 

 subsequent motion which is due to the springs alone corre- 

 sponds to the discharge. It can be seen from the curve, that 

 a large portion of the compression of the springs is given up 

 almost immediately on releasing the weights ; the crossbar 

 then moves to its zero position with gradually decreasing 

 velocity, due to the lower springs recovering their original 

 length very slowly. In fig. 3 the velocity-curve obtained by 

 differentiating curve I. is given. The general form of this 

 curve is very similar to the current-curve one might expect 

 to obtain when charging a condenser ; but in the later parts 

 of the curves the accuracy with which the model imitates the 

 results obtained from actual dielectrics is very striking. In 



