412 Mr. Ervin S. Ferry on the Specific 



no deflexion of the galvanometer-needle was produced when 

 a sheet of ebonite was introduced between the plates of the 

 condenser or when the ebonite was removed. The specific 

 inductive capacity of ebonite is consequently taken to be 2 '62. 



A number of sections of quartz of different thicknesses, 

 from 1 millimetre to 3 millimetres, were cut from several clear 

 crystals. Sections were made both in the direction of the 

 optic axis and normal to the axis. Some sections had approxi- 

 mately parallel faces and some were visibly wedge-shaped : 

 some were polished, some simply smoothed, and some left just 

 as they came from the saw. The mean value of the specific 

 inductive capacity of the sections cut normal to the optic axis 

 was found to be 4*34, and of those cut parallel to the axis 

 4'27. The greatest difference between either of these mean 

 values and of any value actually observed would change simply 

 the figure in the second decimal place. 



In order to determine the change of specific inductive 

 capacity with change of period of the electrical vibration to 

 which the dielectric is subjected, the spark-gap in the primary 

 circuit was opened so wide that no sparks crossed, and hence 

 the electrical oscillation would be produced simply on the 

 make and break of the interrupter of the induction-coil. But 

 with this arrangement there was not sufficient energy induced 

 on the secondary circuits to produce readable galvanometer 

 deflexions even when the condenser was removed from one 

 of the secondary circuits. 



In consequence, an entirely different method was employed 

 to determine the specific inductive capacity with slowly 

 vibrating fields. The method is a very simple one, and no 

 serious trouble was found in its operation. It is simply a 

 Wheatstone bridge with four capacities substituted for resist- 

 ances, the secondary of a very small induction-coil for the 

 battery, and a sensitive telephone for the galvanometer. The 

 same three condensers of the preceding method were used in 

 this, with the addition of another exactly like the standard 

 air-condenser used before. The induction-coil as used would 

 give a spark of about 2 millimetres. It was enclosed in a box, 

 and this box packed in "excelsior" inside of another closed 

 box, so that no sound of the vibrating end was audible. The 

 loudest note in the telephone should be similar to the hum of 

 a mosquito. The ear appears to be most sensitive to changes 

 in this sort of a note. 



The theory of the method is entirely analogous to that of 

 the Wheatstone bridge. If in fig. 2 it be imagined that the 

 four resistances are replaced by four capacities k u k 2 , k z , k±, 

 and if at any moment the potentials at a and c are v and v u 



