PIERCE AND EVANS. — CAPACITY OF CARBORUNDUM. 795 



belt from the motor. The speed of the commutator was measured by- 

 putting a standard mica condenser in the place of the crystal Cr of 

 Figure 1, and reading the galvanometer at G with a known impressed 

 voltage. In practice, in order to eliminate any errors arising from lack 

 of proportionality of the galvanometer deflections, the impressed vol- 

 tage was varied so that the readings of the galvanometer in the speed 

 determinations were brought always to the same deflection (20 cm.), 

 under which condition the number of charges or discharges per second 

 was inversely proportional to the voltage required to give the standard 

 deflection. 



While the galvanometer deflection and voltage were under observa- 

 tion, one of these speeds was found absolutely by means of a revolution- 

 counter attached to the shaft of the commutator, and the other speeds 

 were then known in terms of the speed given by the revolution counter. 



The capacity method of measuring the frequency of charges or dis- 

 charges had the advantage of permitting instantaneous observations of 

 the constancy of the commutator speed before and after measurements 

 with the crystal as condenser. 



Measurements Capable of Repetition. — Although the adjustment of 

 the electrodes upon the crystal so as to get the capacity eifects afforded 

 some difticulty, yet when the adjustments were once made, the galva- 

 nometer readings, under given conditions, were about as constant with 

 the crystal as capacity as with the standard mica condenser as capacity, 

 and all the observations, with ordinary precautions, were capable of 

 repetition with an error of about 1 per cent, which represented the 

 irregularities of the speed of the commutator. 



Galvanometer. — A D'Arsonval Galvanometer of 2000 ohms resist- 

 ance was used. This galvanometer was provided with a heavy closed- 

 coil electromagnetic damper, so as to obtain steady deflections even 

 with comparatively slow speeds of the commutator. High resist- 

 ance or high inductance in the galvanometer used with this method of 

 determining capacity, if the capacity is large, is objectionable, because 

 of the danger of having incomplete charge or discharge. In the present 

 experiments attention was given to the matter of the completeness or 

 incompleteness of the charge or discharge. In one of the experiments 

 described below (Experiment II.) the charges and discharges were prob- 

 ably incomplete, but this was due not to the resistance or inductance 

 of the galvanometer, but to the high resistance of the conducting sheets 

 which served, in the crystal, as armatures of the condenser or else to the 

 high resistance of the contacts of the electrodes with these sheets. 



