calorimeter with water and weigh again. Weigh also the platinoid resistance coil, and support it from the wooden lid of the calorimeter, taking care that it does not touch the sides. Place a thermometer graduated in tenths of degrees in the water. Connect the coil through a standardised ammeter and an adjustable resistance to sufficient storage cells to furnish the current required. Connect a standardised voltmeter of known resistance to the ends of the heating coil. The resistance of the voltmeter is required in order that the current through it may be calculated and subtracted from that indicated by the ammeter. Make circuit and see that the instruments give proper indications and that the thermometer shews a gradual rise of temperature. Break the circuit, stir the water well, and after a few minutes take observations of temperature every half minute as described on p. 129. At the end of the first period put on the current, read the thermometer every half minute, observing the voltmeter 15 seconds before each minute and the ammeter 15 seconds after each minute. This second period should continue till the temperature of the calorimeter has risen about three degrees, then at the end of one of the intervals, the current should be switched off and observation of temperature continued till the rate of change is steady. From the first and third periods the cooling corrections during the second and third periods should be calculated as in pp. 129-130, and from the initial and final corrected temperatures the rise of temperature determined.

J is calculated by substituting in the above equation for E the mean electromotive force, and for A the mean current, if both quantities show only small variations during the experiment. If they are not sufficiently constant, the product of EA must be calculated for each interval of time and the mean product substituted in the equation.

This value happens to be almost exactly right, but errors of one per cent. are likely to occur, unless the voltmeter and ammeter have been carefully standardised.

* The resistance of the heating coil is made small so that the difference of potential between its ends may be small enough to prevent electrolysis of the water.

SECTION LXXI.

INDUCTION OF ELECTRIC CURRENTS.

Apparatus required: Two solenoids of known resistance, one sliding within the other, tangent galvanometer or ammeter reading 1 ampere, reversing switch, mirror galvanometer of low known resistance, resistance coils.

When a current is made, broken, or altered in strength in any circuit, induced currents are produced in neighbouring circuits, and it is the object of this exercise to find on what conditions the magnitudes of these induced currents depend.

The induced currents will last only a very short time, and a galvanometer in one of these neighbouring circuits will not measure the strength of the current, but the total quantity of electricity which has passed through it. If a is the angle of the first swing of the galvanometer needle produced, it is shewn (p. 348) that the quantity of electricity which has passed through the coil of the galvanometer is proportional to sin.

2'

If the galvanometer needle hangs in its proper position when no current passes through the instrument, and the scale is properly adjusted,

11/x

sin =
2 4d 32 a

where is the observed deflection and d the distance of the scale from the mirror (page 160). If an error not exceeding half per cent. is allowable, and the deflections never exceed 25 cms. on a scale placed a metre from the mirror, the second term on the right-hand side is negligible; and we may therefore take the observed reading a to be proportional to the quantity

of electricity which has passed through the galvanometer. The experiments of the present exercise are supposed to be made under these conditions.

Two solenoids, P, S (Fig. 125); mounted on blocks of wood so that their axes are coincident, are provided. The inner coil

Fig. 125.

P can be moved in a direction parallel to the axis, and placed with its centre at any convenient distance from the centre of the outer fixed solenoid S. Each solenoid is divided into three parts, and the number of turns to each part should be counted and recorded.

Arrange the turns of the inner coil in series with each other and with a cell, a reversing switch, an adjustable resistance, and a tangent galvanometer or ammeter. Place it within the outer coil so that the centres of the coils coincide, the mark on its base will then read O on the scale of the outer coil. Connect, by means of copper wire (about no. 18), the end terminals of the outer coil through a resistance box to a low resistance mirror galvanometer. At first cut the mirror galvanometer out of circuit by connecting the two wires leading to it to the same terminal, and observe whether making or breaking the battery circuit has any effect on the galvanometer'. If so remove the coils further away and place their axis in such a direction that this is no longer the case, wherever the inner coil is placed on its slide.

Adjust the resistance in series with the inner coil till the current flowing through it is one ampere, and take readings occasionally to see that it remains steady. Notice that making

1 This is generally unnecessary if the galvanometer is a moving coil instrument.

or breaking the battery circuit produces a deflection of the mirror galvanometer needle. Observe the extent of the first swing and verify that the swing on making is equal and opposite to that on breaking the primary, i.e. the battery circuit, and that the swing on reversing the primary current is double the previous swings. If this is found not to be the case, cut the galvanometer out of circuit as before and make sure that there is no direct action from the primary coil.

Effect of the relative positions of the two coils. Arrange the resistances so that on making or breaking one ampere in the primary, the deflection is about half the greatest observable deflection. Determine its amount, then slide the inner coil through 1 cm. in a direction parallel to the axis of the coils, and again determine the deflection. Repeat for 2, 5, 10, 15 and 20 cms., thus gradually sliding the inner coil out of the outer one.

Place the inner coil on the table, move the outer coil to a distance of about 50 cms., and place it with its centre on the axis of the inner coil produced. Find the direction in which its axis must point in order that there is no deflection on making or breaking the primary circuit.

Move the inner coil towards the outer, keeping it parallel to itself, and verify that it may be adjusted so that there is no deflection even when the coils are near together.

Verify this for positions in which the centre of the outer coil is not on the axis of the inner coil.

Effect of the resistance of the secondary circuit. Replace both coils with the inner coil in the position in which the induced current was found to be a maximum, and determine the swing on making or breaking the primary circuit.

Double the total resistance in the mirror galvanometer (or secondary) circuit by taking out plugs from the box equal to the sum of the resistances of the galvanometer, coil and connecting wires, and verify that the swing is now half what it was before.

Effect of the magnitude of the current in the primary. Increase the resistance in the battery circuit till the current