_=125. Discussion of Experiments 61, 62, 63; Insulators.=_ In 61 the electrification remained at one end of the rod. In 62 and 63 the sparks showed that all parts of the ebonite were not discharged at the same time. A substance, like ebonite, which will not allow electrification to pass from one part of it to another, is called an _insulator_. Silk and glass are also insulators. Do you now see why a silk thread was used to make the carbon electroscope? Why do they fasten telegraph wires to glass insulators? _=126. Conductors.=_ It has already been stated that water in an elevated tank has potential energy. We can allow the water to flow through a conducting pipe to another tank a little lower than the first, and it will still retain much of the potential energy, but not all. Can we conduct from one place to another this peculiar state of things, this queer form of potential energy which we call electrification? It is clear, from the last experiments, that in order to do it we need something besides ebonite, which really acts like a closed stop-cock to the flow of electrification. To keep electrification in one place we need an insulator; to get it from one place to another we need a _conductor_. Insulators are as important as conductors. You saw that sparks went to the finger from the ebonite, so we call the finger a conductor. You have learned that attractions and repulsions show the presence of electrification. Can we have our charged body in one place and get attractions or repulsions at some other place? [Illustration: Fig. 32.] =EXPERIMENT 64. To study conduction.= _Apparatus._ Fig. 32; the support (see § 109); a bent hairpin, H P (No. 39); ebonite sheet, E S; flannel cloth, F C; tin disk, B F B (No. 40), which is the bottom of the flat-box, F B; the insulating table, I T (see § 127). =127. The Insulating Table= consists of a tin box (exactly like that used for the electrophorus cover), and an ebonite rod about 1-3/4 in. long. See § 139 for full details about fitting the rod into the box, etc. The lower end of the short rod fits into the large hole in one end of the support rod, S R. Arrange as in Fig. 32. B F B should swing about very easily. =128. Directions.= (A) Charge E S, then rub it upon I T, as shown, noting the action of B F B. _=129. Discussion.=_ Ebonite being an insulator (§ 125), we say that I T, H P and B F B were _insulated_. You can see that the electrification must have passed through I T and H P to get to the disk, B F B. H P was the _conductor_, allowing the disk, also, to become charged. The wood, S R, is a conductor, and, as it was not insulated from the earth, S R was neutral. Account for the attraction. (See § 121.) [Illustration: Fig. 33.] =EXPERIMENT 65. To study conduction.=