794 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



nected alternately first ■with the charge circuit VG'Cr and then with 

 the discharge circuit CrG. A galvanometer, used for measuring the 

 current out of or into the crystal, could be placed either in the dis- 

 charge circuit at G or in the charge circuit at G'. 



When the galvanometer was placed at G and allowed to come to a 

 steady reading, while the commutator C was in operation, the reading 



^LnjU 



A^-:dc::^B 



FiGtJRE 1. Circuit employed 

 in capacity measurements. 



Figure 2. Clamp for holding spe- 

 cimen between screws A and B. 



of the galvanometer gave the discharge current made up of a series of 

 discharge impulses from the crystal. Placed at G', the steady deflec- 

 tion of the galvanometer, with the commutator in operation, gave the 

 charge current made up of charge impulses going into the crystal. 



The Commutator. — The commutator was made of cold-rolled copper 

 and was used with brushes of the same material so as to avoid thermo- 

 electric currents in this part of the apparatus. 



In order to minimize the leak current, the segments of the commu- 

 tator and the brushes pressing against them were so arranged as to 

 make the time of open circuit between charge and discharge as small 

 as possible. 



In order to simplify the interpretation of results, the time of contact 

 for charge was made approximately equal to the time of contact for 

 discharge. 



Measurement of Frequency of Commutation. — Various speeds of the 

 commutator were obtained by the use of a series of pulleys of different 

 sizes on the axle of the motor and a corresponding series of different- 

 sized pulleys on the axle of the commutator, which was driven by a 



