Capacity of Solids under varying Fields. 91 



each 2J feet long. These wires were supported near the 

 spark-gap at the ends of glass rods covered with paraffin. 

 The transformer and spark-gap were also carefully insulated 

 by glass and paraffin. These precautions are necessary by 

 reason of the fact that a small leakage of electricity from 

 the surface of the small plates changes the reading in a marked 

 manner. Leakage of electricity, however, could be seen by 

 darkening the room, and could also be detected by a peculiar 

 manner in which the sparks went out. This was carefully 

 looked for and avoided. In making the test with oscillating 

 fields, half of the readings were taken with the transformer 

 in one position and then the other half were taken with the 

 ends of the transformer reversed, and the mean of the two 

 series taken as the true reading. This becomes necessary 

 unless the primary and secondary of the transformer are 

 symmetrically arranged in reference io each other. Indeed 

 it was found that the readings could be perceptibly changed 

 by sliding the secondary coil over the primary into different 

 positions. It is easily seen, however, that if the primary and 

 secondary always have the same relation, and the connexions 

 to the primary are reversed after half the readings are taken, 

 the error is eliminated in the mean of all the readings. 

 The current through the coil w r as at times large and again 

 small, and also occasionally reversed ; but these changes, as 

 expected, did not affect the results. The length of the indi- 

 cator spark-gap could also be changed without affecting the 

 mean reading. Readings for slow fields were taken in sets 

 of ten and averaged. The mean of twenty such sets, taken 

 with slow fields, was 3'097. The greatest departure from 

 the mean was + *09. Finallv, from (17), when rZ^ 11*964, 

 <7 2 = 5'010, ^ = 3-097, we obtain K = 6-254. 



Called K = 6-25. 



The value was next obtained for oscillating fields, no 

 change being made in the apparatus, except to connect in the 

 transformer and approach the balls to the proper distance for 

 sparks to pass and cause oscillations. Readings were taken 

 in sets of twenty each. Following is the average of each of 

 the sets taken : — 



10-982 



10-910 



11-052 



10-970 



The connexions to the transforaier were now reversed : — 



