Fig. 



292 Mr. Faraday's Experimental Researches in Electricity, 



way as to give sufficient strength and render tiie apparatus air- 

 tight there, yet leave as much 

 as possible of the lower part of 

 the shell-lac stem untouched, 

 as an insulation between the 

 ball h and the surrounding 

 sphere a, a. The ball h has 

 a small aperture at n, so that 

 when the apparatus is exhaust- 

 ed of one gas and filled with 

 another, the ball h may itself 

 also be exhausted and filled, 

 that no variation of the gas in 

 the interval o may occur du- 

 ring the course of an experi- 

 ment. 



1189. It will be unnecessary 

 to give the dimensions of all 

 the parts, since the drawing 

 is to a scale of one third : the 

 inner ball has a diameter of 

 2'33 inches, and the surround- 

 ing sphereaninternaldiameter 

 of 3*57 inches. Hence the 

 width of the intervening space, 

 through which the induction 

 is to take place, is 0*62 of an 

 inch ; and the extent of this 

 place or plate, i. e. the surface of 

 a medium sphere, maybe taken 

 as twenty-seven square inches, a quantity considered as suffi- 

 ciently large for the comparison of different substances. Great 

 care was taken in finishing well the inducing surfaces of the 

 ball h and sphere a, a; and no varnish or lacquer was applied 

 to them, or to any part of the metal of the apparatus. 



1190. The attachment and adjustment of the shell-lac stem 

 was a matter requiring considerable care, especially as, in con- 

 sequence of its cracking, it had frequently to be renewed. 

 The best lac was chosen and applied to the wire /, so as to 

 be in good contact with it everywhere, and in perfect conti- 

 nuity throughout its own mass. It was not thinner than is 

 given by scale in the drawing, for when less it frequently 

 cracked within a few hours after its cooling. I think that 

 very slow cooling or annealing improved its quality in this re- 

 pect. The collar ^ was made as thai as could be, that the 



Jac might be as large there as possible. In order that at every 



