Oscillatory .Discharges. t>39 



static electrometer, this being very suitable for an accurate 

 determination o£ high potentials. We, however, added to the 

 needle of this electrometer (made out of a thin aluminium 

 plate) a light electromagnetic damper, formed of a copper- 

 plate frame, moving in a magnetic field, and of a form quite 

 similar to that of a Despretz-D' Arson val galvanometer. 



The needle and the damper were very light, their move- 

 ment being nearly aperiodic, so as to allow of following the 

 rapid variations of the potential. 



As a matter of course, the necessary precautions have been 

 taken in order to protect the damper and the mirror against 

 the electrostatic actions. 



Method of securing Constancy of Potential. 



40. The electrodes we used for the discharge, after various 

 endeavours, had the form of small cylinders 3 mm. in dia- 

 meter, terminated by spherical calottes. It proved very 

 difficult, on account of the smallness of the electrodes, to 

 secure a fairly constant potential of discharge for each series 

 of experiments. With aluminium, cadmium, iron, &c. elec- 

 trodes, the alterations of the surface were so rapid, and the 

 delay of discharge, when the sparks passed in the interior of 

 the calorimeter, was so great and so variable, as to make it 

 impossible to ascribe to the mean deviation a reliable value. 

 In the case where we had to complete calorimetric measure- 

 ments, we hence found it very convenient to use platinum, and 

 still better platino-iridium electrodes, also in order to avoid 

 oxidation phenomena. 



In order to diminish greatly the delay of discharge, we 

 put in the interior of the spark-calorimeter, or at a slight 

 distance from the spark-gap, when the photograph was 

 taken, a small celluloid tube containing a little radioactive 

 substance. The action of this substance was so strong that 

 the successive discharge-potentials were very slightly different 

 from one another, and allowed of obtaining a good average 

 tor the value of the discharge-potential, provided the surface 

 of the electrodes was not too much altered. 



In any case it was necessary the sparks should occur with 

 sufficient slowness to allow of the needle following the 

 variations of the potential, in order to have reliable calori- 

 meter-readings. On this account, we were not able to make 

 in each case the readings themselves simultaneously with the 

 calorimeter-readings, for which it was necessary that the 

 sparks should follow each other with sufficient rapidity. YVe 

 therefore made in many cases the determinations of the 

 discharge-potentials immediately before and after each series 



