88 Mr. E. F. Northrup on the Specific Inductive 



method of the values of K under slowly changing and oscil- 

 lating fields is possible, without changing the plates. How- 

 ever, if the capacities are equal for the two sides when slowly 

 changing fields are used, it was found by a careful test that 



d 



the simple formula, K= — } — —5 , may still be used when the 



Ct 2 CL4 



field is oscillating, although the capacities of the two sides 

 are now slightly different. The reason for this is that with 

 oscillating fields the average surface-densities upon the two 

 sides of the centre plate are the same, and hence cancel out 

 of formula (3). The proof of this rests upon the fact that K 

 was found to be the same for all positions of d±, and this 

 would not be the case if the formula were incorrect. Thus, 

 as shown, 



rr die* 



ad 2 — <?'d 4 

 Let a = <7'±Aa', then 



X- dx(T ' - dl (21) 



Now whenever c/ 4 is changed d 2 must change. Hence if A<r' 

 has any value, K must change with d± if calculated by the 



formula K= -, — ^-y, but as a series of tests showed, using the 



"2 ~~ « 4 

 simple formula to calculate the value of K, that its value did 



not change, Ao-' = 0, a — a'; and hence under the conditions 



stated, the simple formula, K=-t — ~ri mav De usea *. 



a 2 — - ^4 



Use of the Apparatus according to the Second Method. 



The disposition of the apparatus discussed under Case II. 

 admits of results being obtained more quickly and satisfac- 

 torily than in the one just described, and should be preferred 

 to the other if two equal slabs of dielectric are obtainable. 

 Headings are taken in the same manner as with the other 

 disposition of the apparatus. The results may be tested in a 

 variety of ways which are readily suggested by the above 

 discussion. 



Observations upon Paraffin. 



The first substance experimented upon was hard paraffin- 

 wax. The block used was a little larger than the plates and 

 4*34 centim. thick. Its melting-point w r as determined to be 

 54£° Centigrade. Only one suitable block was readily ob- 

 tainable, and so the first method was employed. Previously 



