530 Messrs. Trouton and Lilly on the Specific 



the plates so as to avoid changing all edge effects, 

 Fa;= {Ea + E 1 '&-E 1 (a + &)}fo, 



where I is the dimension of the sheet, 

 supposed rectangular, perpendicular to the 

 paper. 



To find in terms of Y and K, we have 



Fig. 1, 



Bi=£/A V-sJ/rt 



E= — P 



=r- 87T 7 ' 



and 



/K^Kj, V=/ a (a + 6) =/a +/\'b, 



where/,/!, and// are the electric forces 

 in the sheet, in the air between the plates, 

 and in the air between the sheet and the 

 plates, Kj being the specific inductive 

 capacity of air. 



Then 



F = 



V^K-KQqZ 

 8ttU+ jjr-b\a + b) 



The form of apparatus we employed in our experiments had 



the condenser plates horizontal 

 The dielectric sheet was formed 

 into a double fan-shaped needle, 

 and supported horizontally by a 

 bifilar suspension. Two pairs of 

 condenser-plates, as shown in 

 fig. 2, were used so as to insure 

 a torsional couple, the two upper 

 and the two lower being for con- 

 venience connected together. 



It is of course immaterial 

 whether the upper plates are po- 

 sitive or negative, the deflexion 

 always being in the same direc- 

 tion. In making the experiment 

 the condenser-plates are con- 

 nected up with a source of elec- 

 tricity, such as a battery of 

 storage-cells, and the deflexion 

 of the needle on coming to rest 

 noted by means of a mirror and 

 spot of light in the usual way. 

 To find the torsional moment in 

 terms of the electric quantities 

 involved, the force on each ele- 

 ment of edge area of the needle 



Fiff. 2. 



+ f rtl N 



\y//y/^'^zx00tiC0\ 



Elevation. 



<^\ 



Plan, 



