_=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.) [Illustration: Fig. 40.] =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? _=151. Discussion.=_ The carbon and E C were insulated, while the hand was "grounded"--that is, it was connected with the earth. Carbon is a good conductor; it may be quickly charged and discharged. =EXPERIMENT 73. To ascertain the location of the charge upon an electrified conductor.= _Apparatus._ The electrophorus (Exp. 68); the insulating table, I T (Exp. 64); the tin box, T B (No. 47), Fig. 41; a piece of moist cotton thread, C T, 5 or 6 in. long, bent double, and hung over the edge of the open box, T B. One-half of C T should be inside of T B, which, in turn, should stand on I T. [Illustration: Fig. 41.] =152. Directions.= (A) Charge E C; pass a spark to T B, and note the action of both parts of C T. _=153. Hollow and Solid Conductors.=_ The moist thread, being a conductor, became charged as well as the box. The electrification seemed to be entirely on the outside of T B. A hollow conductor will hold as large a charge as a solid one having the same amount of surface. This refers to charges of static electricity, not to currents. An electric current passes through the whole substance of a conductor. =EXPERIMENT 74. To study the effect of points upon a charged conductor.= _Apparatus._ The electrophorus (Fig. 34); a pin, bent slightly to keep it from rolling. =154. Directions.= (A) Charge E C; test its charge with your knuckle. Be sure that you get a good spark. (B) Charge E C again, and hold it by its insulating handle, E R, long enough to count 10 before discharging it with your knuckle. Be sure that it holds its charge during this time. (C) While E C is upon E S (Fig. 34), lay the bent pin upon E C, so that its point will project into the air. The point should stick out about 1/4 in. from the edge of E C.