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



the present as midway between the outside plates, they are 

 always at the same potential with respect to each other. 



If a slab of dielectric be now placed upon the centre plate 

 and beneath the movable plate, it will have two effects : 1st, 

 to make the capacity upon its side of the centre plate greater 

 than it is upon the other side ; and, 2nd, to raise the potential 

 at any point above it to a higher value than is the potential 

 at a corresponding point upon the opposite side of the middle 

 plate. Therefore, in order to keep the two small plates at an 

 equal potential in reference to each other, they must be 

 placed at unequal distances from the centre plate. The value 

 of the specific inductive capacity of a substance is deduced in 

 a manner presently to be shown, from the relative positions 

 of these plates. Either one or two slabs of dielectric may be 

 used, and several different dispositions may be given to the 

 apparatus in a manner serving to prove the correctness of the 

 determinations. 



As the object of the experiment is to make a comparison 

 of the values of the specific inductive capacity when obtained 

 under slowly and rapidly oscillating fields, it becomes necessary, 

 in obtaining the value under the latter condition, to completely 

 separate out any slow changes in potential, which the centre 

 plate may be subject to, from the rapid changes which take 

 place when oscillation occurs, and which alone are to be made 

 use of. It follows as a consequence of the large self-induction 

 of the secondary of the induction-coil, that the potential of 

 the plate to which the coil is attached rises slowly at first, 

 until, finally, it becomes sufficiently high to break down the 

 dielectric of the spark-gap ; the rapid oscillations occurring 

 only during the passage of the sparks. The following method 

 was successfully employed to tell when the movable plates 

 had the same potential in reference to each other, and to 

 " weed " out the effects of the rapid oscillation to be used 

 from any effects of slow changes in potential which might 

 take place. 



Two fine wires were led from the movable plates to the 

 primary of a highly insulated transformer which was distant 

 about 2 J feet from the apparatus (figs. 1 and 2). The 

 secondary of the transformer had its terminals end in a spark- 

 gap. This was made by approximating the points of two 

 needles, fastened to the end of a small slab of paraffin held in 

 a wooden clamp. One needle was made to slide in a fine 

 glass tube, to allow the length of the spark-gap to be readily 

 adjusted. It may be mentioned here that any device which has 

 an appreciable capacity as compared with that of the movable 

 plates cannot be employed to show when these plates have 



