=139. Our Electrophorus= is shown in Fig. 34. For the insulating _handle_ use the ebonite rod, E R (No. 28); for the _plate_, use the ebonite sheet, E S (No. 26). The _electrophorus cover_, E C (No. 42), furnished, is a tin box with a fancy top. A hole has been punched in the center of its top, and into the hole has been riveted a short tube, so that the handle, E R, can be firmly held. The hole has been made a little larger than E R for convenience. To make E R fit tightly in the hole, so that you can lift E C, wrap a small piece of paper around the end of E R before pushing it into the hole. You can easily find out how much paper to use to make a good fit. With a knife cut away all loose points of paper that stick out of the hole around E R; this is _important_. The top and bottom of E C should be pressed firmly together. First learn how to use the electrophorus. With the large amount of electrification produced we can then find out how it works. =EXPERIMENT 68. To learn how to use the electrophorus.= _Apparatus._ Shown in Figs. 34, 35. _Do not fail to read_ § 139. =140. Directions.= (A) Place E S upon a _flat_, uncovered, wooden table, and rub it _vigorously_ for a _minute_ with the _warm_ flannel, F C, to thoroughly charge it. Do not let E S slide about, and do not lift it from the table. (B) With the right hand grasp E R at its extreme end, and place E C upon E S. (C) Touch E C for an instant with a finger of your left hand (Fig. 35). (D) Remove your finger entirely from E C, then lift E C by its insulating handle, E R, at the same time holding E S down to the table, if it tries to follow E C. [Illustration: Fig. 35.] [Illustration: Fig. 36.] (E) Bring your left hand near E C (Fig. 36). You should get a good spark from E C. (F) It is not necessary to immediately rub E S again. You have discharged E C by taking a spark from it. To _recharge_ it, simply place it upon E S again; let it remain there while you count 5; touch it as before, and then lift by E R. =141. Extra Notes.= You may repeat the above operation many times. As soon as the sparks begin to get small, electrify E S again. The charge on E C is +, although that on E S is -. You will understand, later, why this is so. =If you do not get a good spark= from the electrophorus, read the directions again. The ebonite must be well electrified; the cover must be lifted by the _end_ of its handle; you must _touch_ the cover and _withdraw your finger_ from it _before_ lifting. You must allow the cover to remain upon the ebonite 3 or 4 seconds each time. The board, or table, upon which E S rests, must be _flat_, and not warped, so that E C will fit down perfectly upon E S. =EXPERIMENT 69. To study "charging by conduction."=