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and r is a fairly large resistance. R should be about 20 ohms. G and A are a galvanometer and an ammeter, respectively. The current going through R divides, part going through G and part

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through S. By adjusting r to the right value the deflection of the galvanometer can be made one centimeter when A reads one tenth of an ampere. The student is to make this adjustment and record the value of r obtained. Give R different values, and record in tabular form the galvanometer readings and the corresponding ammeter readings. The construction of the ammeter, A, is exactly like the galvanometer and shunt arrangement.

(b) The voltmeter.-Connect one side of a very high resistance to one terminal of the galvanometer. Call the other terminal of the resistance A, and the free terminal of the galvanometer B. This arrangement is equivalent to the ordinary voltmeter, but owing to the high sensibility of the galvanometer it will be necessary to put a shunt across the galvanometer terminals in order to decrease its sensibility. Use the specially prepared "voltmeter shunt."

Connect the terminals A and B to a gravity cell; also connect a voltmeter across A and B. Adjust the high resistance so that the galvanometer deflection is exactly ten times as large as the voltmeter reading. Knowing this fact, the voltage indicated. by any deflection can be readily computed.

Remove the voltmeter and cell and connect in a dry cell. Record the voltage indicated by the galvanometer and compare with the value given by the voltmeter. Repeat with a LeClanche cell or any other cell desired.

(c) The Millivoltmeter. (This part need not be worked unless special instructions are given to do so.) It is sometimes desirable to measure very small voltages, such as those produced by thermocouples. The galvanometer may be calibrated to give

values in millivolts (thousandths of a volt). The method used in (a) and (b) consisted merely of comparing the galvanometer indications with corresponding readings of an ammeter or voltmeter. In the present case we make use of Ohm's Law and the laws of resistances in parallel and series and compute the voltages corresponding to the various deflections.

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Connect the galvanometer, resistance boxes, cell and voltmeter as in Fig. 13. Keep R2 at one ohm, and vary R, from several thousand ohms down, so as to obtain deflections of approximately 3, 5, 10, 15, 20 and 25 cm. on the galvanometer scale. Record the actual deflections, the resistances in the two boxes and the voltmeter readings. The latter gives E, the potential drop over the resistances, under the various working conditions. E will change but little. The potential differences across the galvanometer are given quite closely by the following formula:

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Plot a graph with Vg in millivolts, as computed by the formula, for ordinates, and galvanometer deflections as abscissæ. This should be a straight line. From the graph may be read off the voltages indicated by any deflection within the range of the instrument.

EXPERIMENT NO. 412

INDUCED ELECTROMOTIVE FORCES AND CURRENTS.

References: Stewart, Physics, Sect. 472-476, 480-481, 487; Kimball, College Physics, Sect. 707-710, 713-717, 719, 722, 748, 749; Duff, College Physics, Sect. 355-362, 374; Spinney, Text-Book of Physics, Sect. 355-359, 363, 369.

In this exercise a galvanometer is used to indicate the existence and directions of the currents induced in the various circuits described. In order to determine the current directions from galvanometer deflections, it is necessary to test the instrument with currents of known direction and note the resulting deflections.

(a) Connect a short piece of copper wire across the terminals of the galvanometer, then connect the shunted instument with a dry cell and a tapping key. Merely TAP the key and notice the direction of deflection. The current leaves the central, carbon terminal of the cell, hence the binding post by which the current enters the galvanometer may be determined. Record the results, i. e. tell which terminal of the galvanometer the current enters when the deflection is to the right. In the following tests it will be possible to tell which way the current flows in the galvanometer and then to trace its direction through the rest of the circuit.

(b) Two coils are provided, one of many turns of fine wire and another of fewer turns of coarse wire which will slip inside the first coil. Connect the terminals of the outer coil directly to the galvanometer. (Remove the shunt). Plunge the northseeking pole of a bar magnet into the coil and record the action of the galvanometer. Does this effect continue after the magnet stops moving? Draw out the magnet and note the effect. Repeat the tests with the south-seeking pole.

Plunge the north-seeking pole into the coil very quickly, then move it in very slowly, and describe the difference in effect. Thus far in the experiment the wire of the coils has been cutting the magnetic lines of the field. From your observations, what determines the direction of the current? The magnitude of the current?

Using the data from (a) determine what must be the polarity of the coil during each of the above operations and draw sketches showing the relative positions of poles of magnet and coil as each pole of the magnet is inserted and withdrawn.

(c) Now connect a battery through a tapping key to the inner coil and set this coil inside the one which is connected to the galvanometer. Note what happens when the key is closed and when it is opened. Repeat this operation with an iron rod in the inner coil. What is the effect of the iron?

(d) Now close the battery key and draw the inner coil out of the other. Then bring it back again, each time observing the effect upon the galvanometer. Interchange the battery terminals and repeat parts (c) and (d). What change does this produce? In parts (c) and (d) a magnetic field was produced by the current in the inner coil. What general method was used in the three parts for setting up the current in the outer coil? Any current produced in this way is called an induced current.

(e) Connect a lamp in series with a coil of wire to the 110-volt DC circuit. Note the effect upon the brightness of the light when an iron core is inserted in the coil. Explain. Repeat with 110-volt A C. Explain effect. Look up theater dimmers and choke-coils, and see how these effects are utilized.

In the discussion no mention has been made of the induced electromotive forces, because the indicating tests have to be made by means of the induced currents. The student must remember that the currents observed are due to the induced electromotive forces and that the electromotive forces would exist, under the proper experimental conditions, even if the circuits were open so that no currents could flow.

EXPERIMENT NO. 413

THE MOTOR AND THE DYNAMO.

References: Stewart, Physics, Sect. 470, 490, 491; Kimball, College Physics, Sect. 738-743; Duff, College Physics, Sect. 370-372, 376; Spinney, Text-Book of Physics, Sect. 305, 357, 358, 367, 372, 373.

Only the simpler types of direct-current machines will be considered in his exercise. A small dynamo of about 36 watts

power makes a good machine to use. It should be mounted on a wooden base and its field and armature lead-wires should be brought out to conveniently located binding-posts. Direct current from a storage battery or dynamo must be available. Each table-circuit must contain fuses.

(a) The motor. Connect to the direct-current terminals a series circuit containing a switch, a rheostat and the motor with its field and armature connected in series. After the connections are approved, set the motor going, noting especially its direction of rotation. The armature is turned by a current-magneticfield force. What must be done to reverse such a force? What alteration in connections must be made to bring this about? Try out your conclusions. If they prove to be wrong, try again till you succeed in reversing the motor. State definitely what is necessary to reverse a series-wound motor. A series motor has a strong starting torque and is much used on street-cars, hoists and machines where heavy loads must be moved. Note its tendency to speed up indefinitely without a load. A large series motor must not be run without a load.

Change the connections so that the field and armature are in parallel, the rest of the circuit remaining as it was. Devise a method for reversing the motion in this case. State definitely what was done. Shunt motors tend to run at a constant rate even with variable loads, in which respect they differ from the series type; so they find their sphere of usefulness in driving shop and mill machinery where constant speed is desired.

Draw diagrams representing the connections used with both types of motor for both directions of rotation.

If a lamp rheostat is used, try the following experiment. Hold the armature with the hand so as to prevent rotation, and note the brightness of the lamps; then let the motor run, and see if any change in brightness occurs. This will show best with the series motor. What might we expect if we applied full voltage to a large motor at rest? What may happen if too large a load is applied? Look up the meaning of back-or counterelectromotive force and explain this phenomenon.

(b) The dynamo or generator. We may use the same machine as in part (a). Connect the field-coil in series with the rheostat and switch to the direct-current terminals.

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