230 Dr. Fleming and Mr. Ashton on a Model which 



cylinder slightly larger than the springs in diameter. To 

 each spring is attached a piston P,, P 2 which just slides in 

 the cylinder. The two springs with pistons attached are 

 placed one above the other in the cylinder, which is then filled 

 with oil. The top piston Pj must have holes drilled in it 

 sufficiently large to allow the piston to slide very easily 

 through the oil ; let W be a weight just sufficient to compress 

 Sjl and S 2 to half their original length. If the weight be 

 applied momentarily, compression is produced in the upper 

 spring Si ; but provided the weight is immediately removed, 

 the compression of S 2 is infinitesimal. If, however, the 

 weight is kept on, the upper spring Sj is soon compressed to 

 its lull extent ; but the compression of S 2 proceeds very slowly 

 and with decreasing velocity. If the weight is removed, S x 

 regains its original length almost immediately, but the piston 

 P 2 moves up to its original position very slowly. The 

 variation of the displacement of Pj is therefore very similar 

 to the variation in the quantities absorbed and discharged 

 by a condenser when both the charging and discharging 

 are continued for a considerable time. 



3. A simple apparatus such as that described above would 

 draw curves somewhat similar to those obtained from the 

 model about to be described ; but it is evident that these 

 curves would not completely represent the behaviour of a 

 condenser. For the motion of Pj alter the first few seconds 

 of discharge is simply a copy of the motion of P 2 ; also the 

 motion of P 2 at any instant depends solely on the amount of 

 compression in S 2 at that instant. Therefore it follows that, 

 the springs having been compressed to a given extent and 

 released, the motion of P 2 , and therefore of Pj also after the 

 first few seconds, depends only on the amount of the dis- 

 placement, and not on the manner in which that displacement 

 was produced. Thus it will be seen that only one velocity- 

 curve could be obtained from this model, and the equation of 

 the curve would not vary with the time of charge. It has 

 been found that if as many as six pistons and springs are used 

 the curves drawn by the model may be represented, at least 

 approximately, by equations which are similar in form to 

 those obtained by experiments on dielectrics. In the model 

 now described the springs are 2|- inches long, made of 

 No. 12's S.W.G-. brass wire, with a pitch of about 3 inches. 

 To each spring is fixed a brass disk which forms the corre- 

 sponding piston. The piston P 6 fits fairly tight in the 

 cylinder (see figure), and has no hole drilled in it. The 

 second piston has no hole bored in it, but fits slacker in the 

 cylinder than the first. The third piston has one hole ^ inch 



