8G Mr. E. F. Northrup on the Specific Inductive 



the plate is proportional to the capacity of the corresponding 

 side of the apparatus. This, however, is not strictly true. As 

 the alternations grow more rapid, the average surface-densities 

 upon the two sides of the middle plate tend to become equal, 

 though the capacities of the two sides of the apparatus are 

 unlike. Moreover, if the capacities of the two sides differ 

 largely, there is such an interference of waves, when rapid 

 oscillations are taking place, that no position can be found for 

 the movable plates where their potentials in reference to each 

 other will always be equal, and consequently the sparks in the 

 detector spark-gap cannot be made to disappear. This diffi- 

 culty is not observable if the capacities are made nearly the 

 same. It becomes necessary then, in using the apparatus 

 with one slab of dielectric, to have the capacities of the two 

 sides approximately equal. This adjustment can be made in 

 a manner to be shown presently. 



In taking observations by this method we proceed thus : 

 The plates M and N are first brought into contact with the 

 centre plate and the zero positions upon the scales are deter- 

 mined. The block of dielectric to be tested is then placed 

 upon the plate A, and plate M is brought to rest upon it. 

 The reading upon the vernier scale gives the thickness of the 

 block. Great care, of course, is taken to cut this block so 

 that its two faces are parallel. We may now leave plate M 

 resting upon the dielectric or give it any desired position 

 above it. The wires leading from .N and M are first connected 

 directly to the spark-gap and the balls, p and ?i, are so far sepa- 

 rated that no spark can pass between them. In the experi- 

 ment as tried, two large coils were connected in series so as 

 to make the period of the slowly changing field as long as 

 possible. The coil being set in action, the sparks at the 

 detector spark-gap are observed, and the plate N is slowly 

 moved up and down until a point is found where the sparks 

 entirely disappear. There is a distance of about 2 millim. 

 through which the plate may be moved without the sparks 

 appearing. A reading is taken at the upper and at the low^er 

 limit of this space, and the mean taken as the true reading. 

 The limits of accuracy of the method are confined to the 

 accuracy with which this setting of the plate N can be made. 

 Any other error should be attributed to crudeness in the con- 

 struction of the apparatus, and not be considered as intrinsic 

 to the method. These readings so taken, give by applying 

 formula (8) an approximate value of K for slowly changing 

 fields. The capacities of the tw T o sides may now be made 

 nearly equal by approaching the two lower plates until they 

 are the proper distance apart. This distance is determined 



