tus, should always be
applied at the same part of the ball, as, for instance, the summit _k_, and
always in the same way; variable induction from the vicinity of the head,
hands, &c. being avoided, and the ball after contact being withdrawn
upwards in a regular and constant manner.

1200. As the stem had occasionally to be changed (1190.), and the change
might occasion slight variations in the position of the ball within, I made
such a variation 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 apparatus, 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 very dry air, and the other with
air from over water. Though this produced no change in the result, except
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
perfectly 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 to displace dirt and dust which will gradually attach to
it in the course of experiments. I have found much to depend upon this
precaution, 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 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 st