taken of them, with fairly delicate means at hand in testing, in order to obtain an approximate value for H. The value of this so obtained will be the value which holds in the room the test is made in, and may be somewhat different from the values obtained for the year from tables. Using the same magnet in the two cases, we have by oscillations

and if D the scale-deflection corresponding to an angular motion

of the needle, then

H =

2

NOTE. T = where n the number of transits per second.

n'

Apparatus.-Delicate magnetometer with accessories (p. 337); instrument for taking vibrations (p. 335); micrometer gauge sensitive chemical weighing balance; stop-watch; three or four similar permanently magnetized steel needles about 10 cms. long and cm. diameter.

Observations.-(1) Place the vibration instrument on the magnetometer table and a needle in the stirrup. Bring the needle perfectly to rest in the magnetic meridian, then replace the glass shade carefully so as not to jar the needle, and make some convenient mark on the shade opposite one end.

(2) With all iron and magnets removed to a distance, give to the needle a motion of pure rotation of an arc, not exceeding about 7° or 8° by means of an outside magnet, and note the time in seconds of 100 transits (N) of the end of the needle past the mark. (3) Repeat (2) about three or four times, and take the mean,

(4) Place the magnetometer in position on the table, level it, seeing that the needle is quite free to deflect, and adjust the spot of light to zero by slightly moving the scale if necessary.

(5) Place the same magnetic needle, which in the meanwhile has been carefully guarded against vibration, on the sliding carriage (capable of moving along the magnetometer bench due east and west), with its magnetic axis due east and west. Then set the table so that the centre of the magnet and magnetometer needle are dcms. apart, causing a convenient deflection D scaledivisions of the latter.

(6) Reverse the needle so that the other end now points towards the magnetometer, and at the same distance d note the deflection D again.

(7) Repeat (5) and (6) with the magnet at the same distance d on the other side of the magnetometer, and take the mean of the four deflections D.

(8) Repeat (5)-(7) with the same magnet at about five or six different distances, d.

(9) Repeat (1)-(8) for each of the other magnets, and tabulate all your results as follows:

(10) Carefully weigh and measure the magnets.

Distance,

13. Proof of Ohm's Law
(Galvanometer Method).

Introduction.-The above-named is one of the most important fundamental laws of electricity. It states that the difference of potential (P.D.), which we will call V, at the ends of any conductor at constant temperature and carrying a current C, always

bears a constant ratio to that current. This constant ratio is called the resistance R of that conductor. In symbols, therefore, we

= constant (at constant temperature), or, as it is

V

common to write it, C = Either an electrometer (which R

passes no current) or a high-resistance galvanometer (which will pass only a very small current), may be used to measure the P.D., and in this method the latter will be used. If such an instrument has a very high resistance compared with that between the points to which it is applied, its indications will be a correct measure of the P.D. between those two points. It is important to carefully distinguish between the E.M.F. and P.D. respectively at the terminals of any source of electricity. The E.M.F. is the total force tending to send a current round the whole circuit, whereas the P.D. denotes the available force for the external

FIG. II.

circuit alone, after a certain deduction, which depends on the current, is made for the potential lost in the generator itself of internal resistance b ohms, or we may put E V+ Ch. If now a high-resistance galvanometer is used, it will pass only a very small current C, making the term Cb negligible, whence its deflections will be proportional to E or V.

Apparatus. High-resistance mirror galvanometer, g (p. 281); two variable known resistance boxes, r, and ; reversing switch, S (p. 329); spring tapping-key, K; and three Daniell's cells, B.

Observations.—(1) Connect up as shown in Fig. 11, and adjust the galvanometer needle to zero.

(2) With one Daniell's cell in circuit make 1 = 1 ohm (say) and = 4999 ohms. Press K, and note the deflections 0, and 0, either side of zero by turning S.

(3) Repeat (2) for about six different values of 1, altering

r, each time so as to keep r1+r, constant and equal to 5000 ohms.

(4) Repeat (2) and (3) with two and three cells respectively in circuit, and tabulate as follows:

(5) Plot a curve having values of as ordinates and 1 as abscissæ.

Inferences. State clearly all the inferences which you can draw from the results of your experiment. On what does the constancy of the figures in the last column depend?

14. Proof of Ohm's Law
(Electrometer Method).

Introduction.-The above-named, as already stated in the preceding test, is one of the most important fundamental laws of electricity, and it may be enunciated thus

The difference of potential (P.D.) measured electrostatically, which we will call V, at the ends of any conductor at constant temperature, and carrying a current C, always bears a constant ratio to that current. This constant ratio is called the resistance R of V that conductor. In symbols, therefore, we have R =

C

= constant

(at constant temperature), or, as it is more commonly written,

C

=

V

R

In the present method an electrometer (which passes no current at all) will be used in place of the galvanometer of the preceding one, and it may be used in one or other of two ways: (a) ideostatically, i.e. with the needle maintained at an initial high potential by means of a “dry pile" or other independent suitable source of E.M.F., in which case the deflection caused by placing an E.M.F. across the quadrants & P.D. between them; (b) heterostatically, i.e. with the needle merely connected to one pair of quadrants, in which case the deflection ∞ (P.D.)2. These two relations follow at once from the following formula for the quadrant electrometer :

If N = P.D. between the needle and earth or framework of the instrument,

Q1 = P.D. between one pair of quadrants and earth or frame work of the instrument,

and Q=P.D. between the other pair of quadrants and earth or framework of the instrument,

then the deflection of the spot of light on the scale is—

d∞ (Q1 - Q2){N − }(Q1 + Q2)}

from which we see that d is more nearly & Q1-Q2, and that the sensibility of the instrument becomes greater as N increases. A reversing key should always be used with an electrometer, and should be so arranged that when the instrument is not in use the quadrants are short circuited.

1

Apparatus.-Reflecting electrometer, V (p. 289), and its reversing key, K, (p. 298); source of potential, P, for charging the needle; galvanometer, G (p. 271), merely for indicating the current; battery, B, of about 12 cells of fairly constant E.M.F.; key, K; platinoid resistance, R.

Observations.-(1) Connect up as in Fig. 12, where N is the terminal of the needle of V and Qi, Q2 those of the two pairs of quadrants. If P is a

large flings and waste of time in waiting for the deflection to become steady will be avoided. Note the steady deflection D on V and d on G. Repeat this with 7 to 2, K still being closed, and note the deflections again, using the mean.

(3) Repeat (2), using 4, 6, sively, and calculate the ratio

8, 10, and 12, or more cells succes

D

for each. Tabulate as follows:

d