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



Oscillating Fields. 



d 4 in scale - 

 divisions. 



Number of 

 Headings. 



K'. 







1 



2 

 3 



50 

 50 

 50 

 50 



2-25(3 

 2 244 

 2-254 



2-255 



Total . . 200 Mean . . 2'252 

 Called K'=2-25. 



Thus K for paraffin is 3 per cent, less in oscillating fields 

 than in slowly varying fields. 



Observations upon Glass. 



Plate-glass seems to be the most important substance upon 

 which to experiment, as its specific inductive capacity has 

 been found very different by different observers, and because 

 the values of K found for it are much greater than the square 

 of its index of refraction. The greatest pains were taken to 

 obtain certain results for this substance ; and as the values 

 obtained differ considerably from those found by other ob- 

 servers, it is hoped my readers will pardon the mention of 

 some tedious details. 



The second method was employed as described under 

 Case II. Twelve plates of American plate-glass were used, 

 six plates being placed in each half of the apparatus. By 

 making the proper selection of plates for each side, the two 

 piles were made of the same thickness. The density of this 

 glass was determined with a Jolly balance and found to be 

 2*678. All measurements of dimensions were made with an 

 excellent pair of vernier calipers and repeated a large number 

 of times. Referring to fig. 4 for the meaning of the letters, 

 the following dimensions were employed. These are all given 

 in scale-divisions which equal one half the metric system : — 

 P = 10"21, 4=11-964, ^ 2 =5-010, d A = X - 



In order that there might be no springing of the plates 

 after all had been adjusted, eight ebonite posts were placed 

 at the corners as shown in fig. 4. The plate N was supported 

 upon three thin tubes of ebonite. All surfaces where there 

 could be any harmful leakage of electricity, such as along the 

 supports to the plates M and N, were covered with a thin 

 coating of paraffin. The small plates were connected to the 

 spark-gap or transformer with two No. 36 bare copper wires, 



