INDUCTION APPARATUS — PRECAUTIONS IN ITS USE. 13 



purposely, to the amount of an eighth of an inch (which is far more than ever could 

 occur in practice), but did not find that it sensibly altered the relation of the appa- 

 ratus, or its inductive condition as a whole. Another trial of the apparatus was made 

 as to the effect of dampness in the air, one being filled with veiy dry air, and the 

 other with air from over water. Though this produced no change in the result, ex- 

 cept an occasional tendency to more rapid dissipation, yet the precaution was always 

 taken when working with gases (1290.) to dry them perfectly. 



1201. It is essential that the interior of the apparatus should be perfectli/ free from 

 dust or small loose particles, for these very rapidly lower the charge and interfere on 

 occasions when their presence and action would hardly be expected. To breathe on 

 the interior of the apparatus and wipe it out quietly with a clean silk handkerchief, 

 is an effectual way of removing them ; but then the intrusion of other particles should 

 be carefully guarded against, and a dusty atmosphere should for this and several 

 other reasons be avoided. 



1202. The shell lac stem requires occasionally to be well wiped, to remove, in the 

 first instance, the film of wax and adhering matter which is upon it ; and afterwards 

 t-o displace dirt and dust which will gradually attach to it in the course of experi- 

 ments. I have found much to depend upon this precaution, and a silk handkerchief 

 is the best wiper. 



1203. But wiping and sonie other circumstances tend to give a charge to the sur- 

 face of the shell lac stem. This should be removed, for, if allowed to remain, it very 

 seriously affects the degree of charge given to the carrier ball by the apparatus (1232.). 

 This condition of the stem is best observed by discharging the apparatus, applying 

 the carrier ball to the stem, touching it with the finger, insulating and removing it, 

 and examining whether it has received any charge (by induction) from the stem ; if 

 it has, the stem itself is in a charged state. The best method of removing the charge 

 I have found to be, to cover the finger with a single fold of a silk handkerchief, and 

 breathing on the stem, to wipe it immediately after with the finger, the ball B and its 

 connected wire, &c. being at the same time uninsulated: the wiping place of the silk 

 must not be changed ; it then becomes sufficiently damp not to excite the stem, and 

 is yet dry enough to leave it in a clean and excellent insulating condition. If the air 

 be dusty, it will be found that a single charge of the apparatus will bring on an 

 electric state of the outside of the stem, in consequence of the carrying power of the 

 particles of dust ; whereas in the morning, and in a room which has been left quiet, 

 several experiments can be made in succession without the stem assuming the least 

 degree of charge. 



1204. Experiments should not be made by candle or lamp light except with much 

 care, for flames have great and yet unsteady powers of affecting and dissipating 

 electrical charges. 



1205. As a final observation on the state of the apparatus, they should retain their 

 charge well and uniformly, and alike for both, and at the same time allow of a per- 



