The charged E S will be the source of the electrification. New York will represent the end at which we send the message, so at N. Y. we must have a sending instrument. See [Fig. 33], which explains itself. R B or a silk thread must be used to insulate the sender. Around one leg of W S is twisted one bare end of the conductor, C W.
Boston will represent the end of the line at which the message is received, and there we need a receiving instrument. This is similar to the apparatus described in [Exp. 69], [Fig. 37]. In addition to this, tie the middle of a moist cotton thread that is 6 in. long, to B C ([Fig. 37]), and let its two free ends lie over the top and reach down against the bottom of the tin; that is, on the left-hand side. [Fig. 42] will give you an idea in regard to the looks of the thread; at first, however, it should be close to the bottom of the tin. Twist the other bare end of the copper wire around B C.
When the line is properly constructed and ready for use, both instruments and C W are entirely insulated. Do not let any part of C W touch the table or your clothing.
131. Directions. (A) Touch the insulated sending instrument with the charged ebonite sheet, and watch for any motion in the receiving instrument.
Note. Better results will be obtained by using the charged electrophorus cover as the source of electrification, instead of E S. ([Exp. 68].)
132. Discussion. The action here was like that in the previous experiment, the difference being that a longer conductor was used. Electrification is always looking for some place to get to the earth, just as water will run from a roof to the ground. You will understand more about it a little later. In our apparatus just described, the only way that the earth could be reached was through the wooden rod S R. Do not get the idea that real messages are sent in any such way, or that electricity flows through a wire as water flows through a pipe.
133. Relation between Conductors and Insulators. The above terms are merely relative. Static electricity is easily conducted by dry wood, while Galvanic electricity is practically insulated by it. A substance may be an insulator for currents of low potential, while at the same time it will conduct high potential currents. (See Potential [§ 144].)
134. Electrics and Non-electrics. Bodies like glass, sealing-wax, amber, etc., were called electrics by the first students of electricity, because it was upon these substances that they could easily produce electrification. They called iron and other metals non-electrics, because they could detect no electrification after rubbing them. Can you explain why they did not detect any electrification on metals? Can you devise an experiment to prove that metals may be charged? Do you see any relation between a non-electric and a conductor?