INDUCTION APPARATUS DESCRIBED. 9 



bodies around were avoided. The only difference which could occur in the position 

 of anything within, consisted in the deflexion of the torsion thread from a vertical 

 position, more or less, according to the force of repulsion of the balls; but this 

 was so slight as to cause no interfering diflference in the symmetry of form within 

 the instrument, and gave no error in the amount of torsion force indicated on the 

 graduation above. 



1185. Although the constant angular distance of 30° between the centres of the 

 balls was adopted, and found abundantly sensible, for all ordinary purposes, yet the 

 facility of rendering the instrument far more sensible by diminishing this distance 

 was at perfect command ; the results at different distances being very easily compared 

 with each other either by experiment, or, as they are inversely as the squares of the 

 distances, by calculation. 



1186. The Coulomb balance electrometer requires experience to be understood; 

 but I think it a very valuable instrument in the hands of those who will take pains 

 by practice and attention to learn the precautions needful in its use. Its insulating 

 condition varies with circumstances, and should be examined 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 1°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. 



1187. The inductive apparatus. — My object was to examine inductive action care- 

 fully 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 quantities 

 as should sufl[ice 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 diameter, placed, the smaller within the larger, and con- 

 centric with it ; the interval between the two being the space through which the in- 

 duction was to take place. A section of it is given (Plate I. fig. 1.) on a scale of one 

 half: a, a are the two halves of a brass sphere, with an air-tight joint at h, like that 



ihe Magdeburg hemispheres, made perfectly flush and smooth inside so as to pre- 

 sent no irregularity ; c is a connecting piece by which the apparatus is joined to a 

 good stop-cock d, which is itself attached either to the metallic foot e, or to an air 

 pump. The aperture within the hemisphere at /is very small: ^ is a brass collar 

 fitted to the upper hemisphere, through which the shell lac support of the inner ball 



MDCCCXXXVIII. c 



