Capacity of Solids under Varying Fields. 81 



the same potential, if the spark-gap is used directly connected 

 to the plates ; and such is the case when the apparatus 

 is used with slowly varying fields. When the plates are 

 connected with the primary of the transformer, it is not a 

 disadvantage to add capacity to the secondary. However, 

 after trying a large number of devices such as a telephone, 

 electrometer, electroscope, &c, a spark-gap was found to be 

 the simplest, most accurate, and in all respects the most con- 

 venient arrangement both with slowly changing and oscillating 

 fields. One reason why the spark-gap best serves the purpose 

 is because it requires at least 300 volts to make a spark pass, 

 and consequently small and irregular alterations in potential 

 are not shown. An electrometer-needle, on the other hand, 

 keeps up a continual oscillation which is very confusing. A 

 lens was adjusted in front of the spark-gap, and the room 

 darkened when readings were taken. The transformer which 

 was finally adopted-, after many trials of different forms, con- 

 sisted of fine wire wound upon two glass tubes which slid 

 over each other ; the wire upon the inner one forming the 

 primary, that upon the outer one the secondary. The inside 

 tube was 1^ inches in diameter and 7 inches long. This tube 

 was given a thin coating of paraffin to insure high insulation; 

 and in this was traced, with the aid of a lathe, a uniform spiral 

 groove, there being in all 55 turns, 16 to the inch. No. 42 

 copper wire was wound in this groove, and the ends, soldered 

 to terminal loops of somewhat larger wire, were secured at 

 the ends of the tubes with drops of sealing-wax. The 

 secondary, made in the same manner, consisted of 116 turns 

 of wire of the same size, wound in a groove 35 turns to the 

 inch. When all is in proper adjustment for oscillating fields, 

 and the two small plates are at unequal potentials, bright 

 sparks pass at the detector spark-gap, whenever rapid oscilla- 

 tions in the potential of the centre plate occur. If, however, 

 the two balls (p and n, fig. 1, PI. III.) are so far separated 

 that no sparks can pass between them, the potential of the 

 centre plate is then only varying at a slow rate and no sparks 

 pass at the detector spark-gap, for the change of induction in 

 the secondary is not sufficiently rapid to raise the potential to 

 the 300 volts or more required to break down the dielectric. 

 This point was carefully tested by working the coil — which 

 had a sparking distance of 4 inches — to its full capacity and 

 so placing the movable plates as to be always at the greatest 

 difference of potential in reference to each other. Unless 

 sparks passed between the balls, no sparks whatever passed at 

 the detector spark-gap. But whenever sparks did pass 

 between the balls, so that rapid oscillations were taking place, 

 Phil. Mag. S. 5. Vol. 39. No. 236. Jan. 1895. G 



