418 Messrs. Rowland and Nichols on the 



III. 



In comparing the glass with the crystal plates, the electro- 

 meter was rendered as little sensitive as the ordinary arrange- 

 ment of the instrument without the inductor-plate would allow. 

 The electric absorption of the glass plates for a charge in the 

 battery of two or three sparks from the Harris unit-jar then 

 sufficed, after 20 or 30 seconds contact with the battery and 

 5 seconds discharging time, to give a deflection of about 200 

 scale-divisions, which were millimetres. The quartz and cal- 

 cite plates were then alternately substituted for the glass, the 

 same charge and the same intervals of contact being used, and 

 the resulting deflections noted — two plates of each substance 

 of the same thickness being used. 



The results of the measurements are given in the following 

 Tables, the effect of the glass being called 100. 



(«)• 



Table I. 



April 12, 1880. 



Charge of battery, 2 sparks. 



Contact 30 seconds. 



Glass (1st plate) 100-0 



Quartz (1st plate) 17'1 



(2nd plate) 20-0 



Calcite (1st plate) 0-0 



(2nd plate) 0*0 



(b). 



April 13, 1880. 



Charge of battery, 3 sparks. 

 Contact 20 seconds. 



Glass (1st plate) lOO'O 



Quartz (1st plate) 19'3 



Calcite (1st plate) 0-0 



GO- 



April 14, 1880. 

 Charge 3 sparks. 

 - - Contact 10 seconds. 

 Plates carefully dried by being in 

 • . desiccator over night. 



Glass (1st plate 1 ) 100-0 



Quartz (1st plate) 107 



Calcite ( 1 st plate) O'O 



w>. 



April 22, 1880. 



Charge 2 sparks. 



Contact 30 seconds. 



Plate in desiccator since April 14. 



Glass (2nd plate) lOO'O 



„ (1st plate) 96-3 



Quartz (1st plate) 13'4 



• „ (2nd plate) 121 



Calcite (1st plate)..-....... 0-0 



(2nd plate) ...... 0-0 



Table II. 



May 1. Relative Effects for different Intensities of Charge 



and Time of Contact. 



Charge of 

 battery. 



Material. 



Deflections, in millimetres. 



Contact 

 5 seconds. 



Contact 

 10 seconds. 



Contact 

 20 seconds. 



One spark j 



Glass (1st) 

 Quartz (1st) 

 Calcite (1st) 



1330 



130 



00 



1893 



22-7 



0-0 



225-0 



34-3 



00 



Two sparks \ 



Glass (1st) 

 Quartz (1st) 

 Calcite (1st) 



Off the scale 

 240 

 00 



Off the scale 

 350 

 0-0 



Off the scale 

 50-0 

 00 



