_=174. Discussion.=_ This apparatus is really the electrophorus upside down. It shows very clearly (1) the escape of the - electrification from I T, by the spark; (2) that the attraction between I T and E S is much greater than before, when this - is removed; and (3) it shows the different steps of the inducing and charging process, as described in Exp. 75, and as shown in Figs. 43 and 44. CHAPTER X. CONDENSATION OF ELECTRIFICATION. =EXPERIMENT 82. To find whether a large surface will hold more electrification than a small one.= _Apparatus._ The insulating table (for details, see Exp. 64); a large tin basin or pan (not furnished); the electrophorus (Exp. 68). =175. Directions.= (A) Test the electrophorus and be sure that it is working properly. (B) As in Exp. 70, see how many good sparks I T will take from E C (which should be recharged at each trial) before the potential of I T is raised so that it equals the potential of E C. (C) Carefully set the basin or pan upon I T, then count the number of good sparks you can pass to it from E C (recharged at each trial). Compare the number of sparks necessary to raise the potential of the large surface until it equals that of E C, with the number found in part (B). _=176. Electrical Capacity.=_ It takes more heat to raise the temperature of a gallon of ice-water to the boiling point, than it takes for a quart of ice-water. You have just seen that a large insulated surface will take more sparks from a charged body than a small one, before its potential is raised to that of the small one, and to that of the charging body. We say that a large surface has a greater _capacity_ than a small one, the shape and other conditions being the same. =EXPERIMENT 83. To find whether the capacity of a given conductor can be increased without increasing its size.= _Apparatus._ Fig. 52. Insulating table. I T (Exp. 64); the extra ebonite sheet, E S (No. 27); the complete flat box, F B (No. 40, 41); the charged electrophorus cover, E C (Exp. 68). Arrange, as shown, I T being insulated from the earth by E S. F B should rest upon a wooden table or other large conductor. =177. Directions.= (A) See how many good sparks I T will take from E C. Re-charge E C at each count, and note the relative sizes of the sparks. (B) Discharge I T by touching it with your knuckle. [Illustration: Fig. 52.]