Induction Apparatus described. 291 



1186. The Coulomb balance electrometer requires expe- 

 rience to be understood ; but I think it a very valuable instru- 

 ment in the hands of those who will take pains by practice and 

 attention to learn the precautions needful in its use. Its insu- 

 lating condition varies with circumstances, and should be ex- 

 amined before it is employed in experiments. In an ordinary 

 and fair condition, when the balls were so electrified as to give 

 a repulsive torsion force of 400° at the standard distance of 30°, 

 it took nearly four hours to sink to 50° at the same distance ; 

 the average loss from 400° to 300° being at the rate of 2°*7 

 per minute, from 300° to 200° of l°-7 per minute, from 200° 

 to 100° of l°-3 per minute, and from 100° to 50° of 0°'87 per 

 minute. As a complete measurement by the instrument may 

 be made in much less than a minute, the amount of loss in 

 that time is but small, and can easily be taken into account. 



1 187. The inductive apparatus. — My object was to examine 

 inductive action carefully when taking place through different 

 media, for which purpose it was necessary to subject these 

 media to it in exactly similar circumstances, and in such quan- 

 tities as should suffice to eliminate any variations they might 

 present. The requisites of the apparatus to be constructed 

 were, therefore, that the inducing surfaces of the conductors 

 should have a constant form and state, and be at a constant 

 distance from each other ; and that either solids, fluids, or 

 gases might be placed and retained between these surfaces 

 with readiness and certainty, and for any length of time. 



1188. The apparatus used may be described in general 

 terms as consisting of two metallic spheres of unequal diame- 

 ter, placed, the smaller within the larger, and concentric with 

 it; the interval between the two being the space through 

 which the induction was to take place. A section of it is 

 given (fig. 1.) on a scale of one third: a, «, are the two 

 halves of a brass sphere, with an air-tight joint at b, like that 

 of the Magdeburg hemispheres, made perfectly flush and 

 smooth inside so as to present no irregularity ; c is a connect- 

 ing piece by which the apparatus is joined to a good stop-cock 

 df which is itself attached either to the metallic foot e, or to 

 an air pump. The aperture within the hemisphere aty is very 

 small : ^ is a brass collar fitted to the upper hemisphere, 

 through which the shell lac support of the inner ball 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 I is fitted into the socket g, by a little ordinary resinous 

 cement, more fusible than shell-lac, applied atmmin such a 



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