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on the paper. Magnets about 5 cm long and 0.5 cm in breadth and thickness make very satisfactory fields.

(b) To plot the field of the magnet alone.

In order that the earth's field shall have no effect on the compass needle it is necessary that the needle should always point in the direction of the earth's field. To secure this condition, stretch a string in the direction of the earth's field far enough above the table that the paper, magnet and compass may be moved about under it. Fasten the magnet to the paper with a bit of wax and, starting as before to plot the lines of force, always turn the paper so that the compass stands directly under the string, pointing north. Trace about the same number of lines as before.

In this experiment, explain by vector method the neutral points.

In the second part explain by vectors the reason that the method eliminates the earth's field.

EXPERIMENT NO. 403

ELECTROSTATIC PHENOMENA.

References: Stewart, Physics, Sect. 369-383, 505-507; Kimball, College Physics, Sect. 517-553; Duff, College Physics, Sect. 252-256, 260-263, 267, 273; Spinney, Text-Book of Physics, Sect. 255, 256, 261-266, 269.

While this exercise is in many ways merely a repetition of experiments usually shown on the lecture table, it is commonly of considerable interest to the student to cause these phenomena to take place himself. It will be well to write up each part of this exercise by itself, telling the procedure, the results and the conclusions, and illustrating the distributions of charge by suitable sketches.

(a) Rub one end of a hard-rubber rod with flannel, bring that end near some cork filings and notice what happens. Again rub the rod and suspend it in a horizontal position by means of a thread. Bring near the charged end a second hard-rubber rod which has also been rubbed with flannel and note the effect. Then bring up the flannel just after it has been used for rubbing

the rod, and compare with the preceding result. We define the charge on the rubber as a negative charge and say that the flannel has a positive charge. Formulate the law of action between like charges and between unlike charges.

One should be sure that he has real hard-rubber and not bakelite, which is sometimes sold for hard-rubber. Bakelite is not so good an insulator as hard-rubber and cannot be relied upon to give a negative charge when rubbed with flannel.

(b) With the electroscope uncharged, touch the charged end of a rubber rod to the knob. The electroscope is thus charged by conduction, the rod having given some of its charge to the electroscope. Bring up in turn the charged rod, the charged flannel and the hand, but do not touch them to the knob. Record the effects.

(c) Charge the electroscope. Try the effects of touching the knob with an uncharged rubber rod, a dry piece of wood, a metal object, the finger, and a rubber-covered wire held by the insulation. Recharge whenever necessary. Which objects are conductors and which non-conductors or insulators? Could you detect different degrees of conductivity? Can you rely on the ordinary insulating wrappings of wires to prevent the flow of electricity under the conditions of this experiment, in which the potential differences range from 1000 volts upward?

(d) Bring the charged end of a hard-rubber rod near, but not in contact with, the knob of an uncharged electroscope. An uncharged body is supposed to possess equal positive and negative charges. Using this assumption and the laws of attraction and repulsion of charges, explain the action of the leaves. Remove the rod. What happens and why? Draw diagrams with signs to show the distribution of charges in these tests.

Again bring up the rod and, while it is near the knob, touch the knob momentarily with the finger. Remove the finger and then the rod. Record what happened at each step of the procedure and make sketches showing the distribution of charges on the electroscope after each step has been taken. This method of charging an electroscope is called charging by induction. Does it leave the electroscope with a charge similar to that on the rod, or opposite to it in sign?

(e) In the following parts of the exercise, the metal vessels must be insulated from the table and from each other by thick blocks of paraffin. Connecting wires, even when rubber-covered, must not be allowed to touch the table and must be handled only with hard-rubber rods. A dry silk cord may be used for suspending the charged sphere, mentioned hereafter, but it is also wise to suspend the cord from a hard-rubber rod.

Rub the rod with flannel and charge an insulated conducting sphere by contact. What will be the sign of the charge on the sphere? Lower this sphere into an insulated metal vessel connected with an electroscope and observe the effect on the leaves. Do not touch the vessel with the sphere. Remove the sphere and note the effect. Is the sphere still charged? Lower the sphere into the vessel a second time, and touch it to the vessel, noting carefully at the same time the behavior of the electroscope leaves. Remove the sphere and note the effect this time. Test the sphere for a charge. Make a set of diagrams showing clearly the distribution of the electricity in each of the four steps described above.

(f) Charge the electroscope by induction, using the hardrubber rod rubbed with flannel. This will give the electroscope a positive charge. Rub the hard-rubber rod and flannel together and try the effects on the electroscope of bringing near it, first the positively charged flannel, and second the negatively charged rod. Pass the charged rod rapidly through a Bunsen flame a few times and see if it still has a charge. Does a flame show conducting properties? When an electroscope is POSITIVELY charged, an increase in divergence of its leaves shows an increase in its potential, a collapse of the leaves a decrease. What is the effect on the potential of a body of bringing near it a positively charged object? A negatively charged object? When a body is brought near a charged electroscope, an increased divergence of the leaves is a certain indication that the body has a charge similar to that on the electroscope. A collapse of the leaves is not a certain indication of a dissimilar charge on the body. Note the effect of bringing up the hand in part (b). Using these facts, determine what kind of a charge is produced on sulphur when rubbed with flannel, and on a piece of metal, having an insulating handle, when rubbed with flannel.

EXPERIMENT NO. 404

FALL OF POTENTIAL ALONG A CONDUCTOR.

References: Stewart, Physics, Sect. 408, 412, 414, 441; Kimball, College Physics, Sect. 639-640; Duff, College Physics, Sect. 316, 317, 319; Spinney, Text-Book of Physics, Sect. 316-318.

Ohm's law states that, when a direct current flows along a conductor, the potential drop between any two points is proportional to the resistance between them and to the current which flows. If V is the potential drop, R the resistance, and I the current,

V=RI.

This experiment is designed to show that this relation holds, first, over a uniform conductor and, second, over one which is not uniform. In each case the effect of varying the current is to be tested.

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(a) Tests with a uniform conductor.

Set up a circuit as indicated by Fig. 8. B represents a cell, r1, a small resistance box, XY one or two meters of uniform wire stretched along a board, r2 a high resistance, D a sliding key, and G a galvanometer, whose deflections are read with telescope and scale. A low-range voltmeter may be used in place of the galvanometer, in which case several cells in series must be placed at B.

Adjust the galvanometer so that the telescope is focused on the scale at the zero point. Insert a low resistance at r1, say 10 ohms, and adjust r2 to a high value such that when the sliding key, D, is put at Y the galvanometer reading remains on the scale. In the plug type of resistance box, plugs are removed to insert resistance; in the dial type, the dials are set at the desired figures.

Take the galvanometer readings with the key, D, first at Y and then successively at points 10 cm apart along the wire till X is reached.

Increase r1 fifty per cent, but make no other change, and then take a similar series of readings. Finally take a third series with r1 at twice its original value.

The deflection of the galvanometer is proportional to the difference of potential between X and D. The resistance of a uniform wire is proportional to its length. Judging from the data taken above, what relation would you say exists between the resistance between two points and the potential difference between the points, the current remaining constant?

Plot curves using galvanometer deflections as ordinates and distances, XD, as abscissae. The curves should be straight lines. 'What does this fact signify? Plot the three curves on one diagram.

How does the potential drop vary with the resistance, when the current is constant? How does it vary with the current, when the resistance is constant?

(b) Tests with a non-uniform conductor. To the 110volt D C terminals, connect in series an ammeter, a bank of four or five lamps with a switch, and a slide rheostat, using the end terminals of the last. Set the slide about one third of the way along the tube. After your set-up is approved, unscrew all but one lamp and close the switch. Using a voltmeter of 120 to 150 volts range, take readings as follows: across main terminals, across the lamp bank, from one end of rheostat to slide and from slide to the other end of rheostat. In connecting up the voltmeter, be careful not to touch both sides of the line at the same time nor to bridge any gap in the circuit with your body. Record all voltmeter readings and the current. Repeat with two, three and four lamps turned on in succession.

In each case compare the terminal voltage with the sum of the other three. Does it check in all cases? State the relation between the total potential drop over a line, and the drops over all parts of the line.

As the current increases, how does P. D. over the lamps change? What happens to the P. D. over the two parts of the rheostat at the same time? In the lamp bank the resistance has

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