690 Prof. L. T. More on Dielectric 



some action in the dielectric, but later found that it was 

 caused by charges induced on the rubber which attracted the 

 tilting mirror statically. 



During the final experiments which continued through two 

 months, the rubber sheeting was replaced by pure lard oil. 

 The dielectric constant is almost exactly that of the shellac 

 mixture. The height of the frame was carefully adjusted to 

 make the electrical forces a balanced system, as I have 

 previously shown to be necessary. 



When these precautions were taken the mirror showed 

 absolutely no deflexion when the plates were charged to any 

 potential up to the breaking point for the dielectric. 



The conditions were varied as far as possible. Sometimes 

 the charging was done slowly and sometimes quickly, ranging 

 from five seconds to a minute or more, and at times Leyden 

 jars were included in the circuit and sometimes not, so that 

 the quantity of charge might be varied. 



From the nature of the construction of the condenser 

 surface leakage was obviated, as little or no metallic surface 

 was exposed, and in my former work I showed that surface 

 currents were principally responsible for the heating. The 

 currents through the dielectric are always small, and their 

 effect was so much retarded in time by the large mass of 

 dielectric through which the heat must diffuse, that the con- 

 denser could be charged and discharged before the slow creep 

 of the image due to this occurred. 



Potentials which produced a spark of 4 millimetres between 

 balls 1 centimetre in diameter were first used, and then 

 increased gradually a millimetre at a time until a spark of 

 16 millimetres was reached. This latter corresponds to a 

 potential of about 130 c.G.s. 



In not a single case did the mirror show either an elongation 

 or a contraction. 



When the spark passed, there was usually a slight tremor 

 or jump of the mirror of not more than six or seven divisions 

 of the micrometer, and this was sometimes in the direction 

 of an elongation and sometimes a contraction. These were 

 found to be due to the explosion of the spark transmitted 

 through the air. To prove this, the condenser was dis- 

 connected from the electric circuit and sparks passed between 

 the balls of the micrometer; in every case the same jump of 

 the mirror was observed, although the condenser was not 

 electrified. Shearer mentions this same disturbance while 

 using an interferometer as a measuring device. 



That my apparatus was amply sensitive is seen from the 

 following calculation. 



