148. Directions. (A) Thoroughly charge E C, and test it with your knuckle to be sure that it is working properly.
(B) Charge E C again; hold the pin in your left hand ([Fig. 39]), and slowly bring its head toward E C; listen for sparks.
(C) Recharge E C, and bring the point of the pin slowly toward it. Touch E C to see whether it has been discharged or not.
149. Disruptive, Conductive, and Convective Discharges. Sudden discharges, accompanied by bright sparks, are said to be disruptive. When the electrification is continuously carried away by a conductor, there is a conductive discharge. There is a convective discharge when the electrification escapes from points into the air. (See [§ 155].) The nature of the discharge depends upon the potential of the charge, upon the nature of the charged conductor, and upon the nature of the surrounding air and objects. Convective discharges are often silent, as in [Exp. 71] (C). In this case, electrification passed from the earth through the pin-point to the cover to neutralize it. (See [Induced Electricity].)
EXPERIMENT 72. To study intermittent or step-by-step discharges.
Apparatus. Electrophorus ([Exp. 68]); carbon electroscope ([§ 114]), ([Exp. 58]).
150. Directions. (A) Charge E C, then hold your hand on one side of the carbon ([Fig. 40]), and hold E C upon the opposite side. What should the carbon do?