Conductivity of some Dielectrics. 489 



These modifications, by reducing variations of pressure 

 and enabling their occurrence to be readily detected, render 

 it advantageous to employ a more sensitive galvanometer 

 and a larger specimen than before. Hence both larger and 

 smaller current densities can be dealt with and the range of 

 observations considerably increased. It is also possible to 

 work at higher pressures, which is most important when 

 dealing with insulators that cannot be obtained in sheets as 

 thin as those of mica. 



Details of Apparatus. 



The electrical connexions are shown dia grammatically 

 in fig. 1. I is a " 10 inch " induction coil whose primary 

 carries an alternating current from a 50 volt 50 cycle 

 supply. This can be varied by a set of rheostats from a 

 small fraction of an ampere up to about 7 amperes, and is 

 roughly measured by the ammeter A. V 1 and V 2 are two 

 Ediswan E.S. 12 rectifying valves, V 2 and its filament 

 battery being supported on an insulated stand. The point E 

 is earthed. R is a carbon streak resistance of about 

 2 megohms. This is necessary to prevent an excessive 

 current from passing through the valves if the high tension 

 system is earthed, either intentionally or throngh the failure 

 of a specimen. B is a battery of leyden jars of capacity 

 about 0'017 microfarad. 



C is an air condenser (about 6 x 10~ 4 m.f.) whose outer 

 shell is earthed through the ballistic galvanometer Gr 3 , which, 

 as in the previous arrangement, is protected from high pres- 

 sures by a large condenser in parallel and an inductance in 

 series. It is normally short-circuited by the key D, which 

 is held in the position shown by a trigger. On releasing 

 this, the key moves so as first to break this short circuit, 

 and immediately afterwards earth the high tension system. 

 The resulting throw of the galvanometer measures the pres- 

 sure of the system. This arrangement, which is identical 

 with that previously used, was on this occasion only employed 

 to calibrate another device for the same purpose. 



This, shown diagrammatically at F, consists of a rotating 

 contact-maker which alternately charges a small condenser, 

 and discharges it through a circuit, in which the dead-beat 

 galvanometer G 3 may be included by depressing the Morse 

 key K 2 . As the contact-maker always revolves at the same 

 speed, the average current through Gr 2 measures the average 

 pressure of the source. 



Phil. Mag. S. 6. Vol. 42. No. 250. Oct. 1921. 2 L 



