_Apparatus._ Fig. 37. To one end of a _silk_ thread, S T, is tied a little bent clamp, B C (No. 46); the other end of S T is tied to the support wire, S W (No. 36); the bottom of the flat box, B F B (No. 40), is supported by B C, and thus _insulated_ from the table and earth; the electrophorus (Exp. 68) is also necessary. =142. Directions.= (A) Charge E C (Exp. 68), and bring it near B F B (Fig. 37). Note the spark. (B) Repeat (A) twice, noting the relative sizes of the sparks. Does B F B continue to be attracted by E C? (C) Bring your knuckle slowly towards the charged disk, B F B. [Illustration: Fig. 37.] [Illustration: Fig. 38.] =EXPERIMENT 70. To study potential; electro-motive force.= _Apparatus._ The insulating table, I T, Fig. 38. (For details see Exp. 64; the electrophorus Exp. 68). =143. Directions.= (A) Pass a spark from the thoroughly charged E C (Exp. 68) to I T. (B) Recharge E C, and see how many times I T will take good sparks from it, and note the relative sizes of the sparks. (C) As soon as I T refuses to take more sparks from E C, touch E C to see if it is completely discharged. (D) Touch I T. _=144. Pressure; Potential; Electro-motive Force.=_ Water runs down hill. It always tries to run from a high place to a lower one. Electrification acts very much like water in this respect. We say that water has a _pressure_, or a _head_ of so many feet. In speaking of a charge, we say that it has a _potential, or an electro-motive force_. Water may have a high or low pressure, and a charge may have a high or low potential. The greater the pressure of water, the harder it tries to break away and get somewhere; the greater the potential of a charge, the farther it will jump to your hand. _=144a. Current; Spark.=_ Electrification will easily pass from a place of high potential to one of low potential through a conductor, and when it _passes_ we say we have an _electric current_, or a _current of electricity_. Water has no desire to flow on a dead level, and the electric current does not care to flow between two places of equal potential. The potential of the earth and of all neutral bodies is zero; that is, they have no charge, no potential; so it is very easy for a charge to escape into the earth. Dry air is a pretty good insulator, but when the attraction between a charged and a neutral body gets great enough, the spark rips right through the air. Benjamin Franklin proved by experiment that lightning is caused by the electrification in the clouds and air. (See Atmospheric Electricity.)