296 Mr. Faraday's Experimental Researches in Electricity. 



aqjount 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 apparatus, or its inductive condition as a 

 nxsJiole. Another trial of the apparatus was made as to the 

 effect of dampness in the air, one being filled with very dry 

 air, and the other with air from over water. Though this 

 produced no change in the result, except an occasional tend- 

 ency to more rapid dissipation, yet the precaution was always 

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

 fectly. 



1201. It is essential that the interior of the apparatus should 

 hQ perfectly free from dust or small loose particles, for these 

 very rapidly lower the charge and interfere on occasions when 

 Iheir presence and action would hardly be expected. To 

 bi-eathe 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 care- 

 fully 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 to displace 

 dirt and dust which will gradually attach to it in the course 

 of experiments. I have found much to depend upon this pre- 

 caution, and a silk handkerchief is the best wiper. 



1203. But wiping and some other circumstances tend to 

 give a charge to the surface 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 dis- 

 charging 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 ex- 

 cellent 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 morn- 

 ing, and in a room which has been left quiet, several experl- 



