Читать книгу The Study of Elementary Electricity and Magnetism by Experiment - Thomas M. St. John - Страница 10

Table of Contents

EXPERIMENT 61. To study insulators.

Apparatus. Ebonite rod, E R (No. 28); flannel cloth, F C (No. 30); tissue-paper, T P (No. 31).

122. Directions. (A) Holding one end of E R in the hand, charge the other end by rubbing it with F C.

(B) With bits of the T P test each end of E R for a charge, and compare the results.

EXPERIMENT 62. To study insulators.

Apparatus. The ebonite sheet, E S (No. 26); flannel cloth, F C (No. 30).

123. Directions. (A) Thoroughly electrify E S (Exp. 51, D), then lift and hold it in the air, as in Fig. 28.

(B) By moving your rounded knuckle about near the surface of E S, see if you can get more than one spark from it.

EXPERIMENT 63. To study insulators.

Apparatus. A hard-rubber comb (not furnished); flannel cloth, F C (No. 30); dull pointed nail (No. 19).

124. Directions. (A) Electrify the comb with F C.

(B) Move the nail along near the teeth of the comb, and listen carefully.

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?

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 1¾ 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.)

Fig. 33.

EXPERIMENT 65. To study conduction.

Apparatus. A copper wire, C W (No. 44); insulating rubber band, R B (No. 45, Fig. 33); wire swing, W S (No. 37); the other half of the flat box, T F B (No. 41); apparatus of Exp. 69.

130. Telegraph Line. To have our telegraph line using frictional electricity complete, we must have: (1) Some way of generating or making the electricity; (2) Some means of getting it or its effects to the other end of the line; (3) Some way of showing that it has been taken there.

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?

EXPERIMENT 66. To study the effect of moisture upon an insulator.

Apparatus. Same as for Exp. 65, with the exception of the copper wire; this is to be replaced by a dry silk thread about 2 feet (60 cm.) long (No. 33).

135. Directions. (A) See if a charge can be sent through the thread, in the same manner as it was through the copper. Is dry silk a conductor?

(B) Thoroughly wet the thread, being careful not to wet the rubber band insulator (Fig. 33); see if wet silk is a conductor.

136. Discussion. Dry silk is an insulator, while wet silk is a good conductor of static electricity. It is the water, however, which really does the conducting. Even small amounts of moisture on glass, or other insulators, will allow the charge to escape. Glass collects much moisture from the air. Do you now see why it is necessary, to get good results, to have the paper, glass, etc., hot before electrifying them?

EXPERIMENT 67. To test the effects of moisture upon bodies to be electrified.

Apparatus. Two pieces of newspaper, each about 4 in. (10 cm.) square.

137. Directions. (A) Heat one piece to make it thoroughly dry, and leave the other cold.

(B) Stroke each, say 10 times, with your hand, pressing them upon the table; then place them upon the wall at the same time, being careful not to let them touch your clothing. See which will cling to the wall the longer.