10 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XI.) 



and its stem passes ; h is the inner ball, also of brass ; it screws on to a brass 

 stem i, terminated above by a brass ball B ; /, / is a mass of shell lac, moulded carefully 

 on to i, and serving both to support and insulate it and its balls h, B. The shell- lac 

 stem / is fitted into the socket g, by a little ordinary resinous cement, more fusible 

 than shell-lac, applied at m m in such a way as to give sufficient strength and render 

 the 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 sur- 

 rounding sphere a, a. The ball h has a small aperture at w, so that when the appa- 

 ratus is exhausted 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 daring 

 the course of an experiment. 



1189. It will be unnecessary to give the dimensions of all the parts, since the 

 drawing is to a scale of one half: the inner ball has a diameter of 2*33 inches, and 

 the surrounding sphere an internal diameter 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, may be taken 

 as twenty-seven square inches, a quantity considered as sufficiently large for the com- 

 parison 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 requi- 

 ring considerable care, especially as, in consequence 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 continuity 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 an- 

 nealing improved its quality in this respect. The collar g was made as thin as could 

 be, that the lac might be as large there as possible. In order that at every re-attach- 

 ment of the stem to the upper hemisphere the ball h might have the same relative 

 position, a gauge /? (fig. 2.) was made of wood, and this being applied to the ball and 

 hemisphere whilst the cement at m was still soft, the bearings of the ball at q q, and 

 the hemisphere at r r, were forced home, and the whole left until cold. Thus all dif- 

 ficulty in the adjustment of the ball in the sphere was avoided. 



1191. I had occasion at first to attach the stem to the socket by other means, as a 

 band of paper or a plugging of white silk thread ; but these were very inferior to the 

 cement, interfering much with the insulating power of the apparatus. 



1192. The retentive power of this apparatus was, when in good condition, better 

 than that of the electrometer (1186.), i. e. the proportion of loss of power was less. 

 Thus when the apparatus was electrified, and also the balls in the electrometer, to 

 such a degree, that after the inner ball had been in contact with the top k of the 

 ball of the apparatus, it caused a repulsion indicated by 600° of torsion force, then 



