position, a precaution of practically no consequence if the needle be pivoted and the controlling force be due to the earth's magnetism alone, but desirable if the whole or part of the controlling force is produced by a not very distant magnet. Of course the magnet must be so far away that neither the magnitude nor direction of its attraction on the suspended needle is altered by the turning of the needle; but this need not be very far, unless the needle employed is long. v is a vessel containing two zinc plates for adjusting the strength of the current by placing the plates at different distances apart, or, instead, a V-shaped tube containing zinc rods as seen in Fig. 56, page 109, can be more conveniently employed for this purpose. w is one of the wires leading to the current generator, and T is the terminal to which the other wire is attached.

To perform the calibration, the galvanometer is placed as it is intended to be subsequently used, and a current sent through it producing a deflection, d1, say; then the galvanometer is turned round until the pointer points to some fixed mark on the scale, the current stopped and the angle a, observed through which the pointer moves from this mark. Next the galvanometer

is replaced in its normal position, a second current sent through it, producing a deflection da, say; then the galvanometer is turned round as before until the pointer points to the same fixed mark, the current stopped, and the angle a, observed through which the pointer moves from the mark. This process having been carried out with various currents, it follows from what precedes that the currents which produce the deflections d1, d2, d2, &c., when the galvanometer is kept fixed in its normal position are proportional to sin. α1, sin. sin. &c.

In certain cases, even although the needle be suspended by means of a silk fibre, it may be possible to pivot the galvanometer, so that it may be easily turned, in a horizontal plane, about the axis of suspension of the needle without setting the needle swinging. Under

these circumstances the method of turning the coil so as to follow the needle may be conveniently employed. In Fig. 64 is seen the section of a galvanometer showing one method of pivoting.

The galvanometer is of the type described in § 33, page 125, and illustrated in Fig. 65, the section, seen in

Fig. 64, passing through the centre of the instrument transverse to the coils c, c, which are fixed to the base b. A screw, s, fastens the base b of the galvanometer to the base-board B, but leaves it free to be turned, the bearing surfaces being those of the brass pieces 81 and 8.29 attached respectively to b and B. The top of s, is filed flat, and a washer inserted under the head of the screw s prevents this screw from being tightened or loosened when the galvanometer is turned about it.

If the needle be suspended by means of a silk fibre and hang very close to the wires of which the coil is made, which is always the case with a very sensitive galvanometer, the instrument must be carefully levelled with levelling screws until the needle hangs free of the coil. In such a case it would be quite impossible to turn the instrument round unless its construction were specially designed for that purpose. If then

the sine principle be applied to an ordinary sensitive galvanometer the second method (page 113) must be employed.

The controlling magnet must be put sufficiently far away from the galvanometer to produce a field that is uniform throughout the small space in which the needle moves, and must be sufficiently powerful that the entire control of the needle is practically produced by this magnet alone, that is to say, if the controlling magnet be placed at the distance l, say at which it is to be used, and with its axis in any radial line passing through the centre of the suspended needle, the needle must place itself in this radial line when no current is passing through the galvanometer.

The two conditions referred to at the beginning of the last paragraph can be most easily fulfilled if before commencing the experiment the magnetic field due to the earth-iron gas-pipes, &c.-be neutralised by a magnet suitably placed, and left untouched during the calibration.

A circle of radius 7, and with its centre in the vertical line about which the needle turns, having then been drawn on the table, the controlling magnet is placed on the table with its centre at different points on the circumference of this circle, and with its axis in a radial line passing through the vertical line about which the galvanometer needle turns; then the currents C1, C2, C3, &c., which are required respectively to bring the needle to some fixed position, are respectively proportional to the sines of the angles a1, ag, ag, &c., which the galvanometer

needle makes with the axis of the controlling magnet in its different positions.

This novel method of applying the sine principle to the calibration of a sensitive galvanometer, the construction of which did not allow of the galvanometer being moved while in use, has been employed with success in the laboratories of the Central Technical College.

To calibrate a galvanometer by the employment of the sine principle requires the current in each case to remain constant long enough for the instrument to be turned round after the needle, or for the direction of the controlling force to be altered, until the needle and the deflecting force are brought to a fixed position relatively to one another. But when once the calibration curve has been drawn, a galvanometer so calibrated can, of course, be used to measure currents as transient as a galvanometer calibrated in any other way.

33. Construction of Galvanometers in which the Angular Deflection is directly Proportional to the Current. We have already seen (page 84) that the current is proportional to the tangent of the deflection of the galvanometer needle, when neither the magnitude nor direction of the controlling force is altered as the needle moves into a new position on being deflected, and when, in addition, the direction of the controlling force is at right angles to the direction of the force with which the current passing round the coil acts on the needle.

In order, therefore, that the angular deflection may be directly proportional to the current, we must either cause the needle on being deflected to move into a position in which the current passing round the coil acts more powerfully on it, or into a position in which the controlling force becomes weaker; or we may arrange that both these results may be produced.

The first condition may be obtained in a rough way by employing the very defect of construction previously

referred to in the adjustment of the tangent galvanometer, which made the deflection on one side of the zero larger than that produced by the same current on the other—viz., not putting the coil so that its plane was parallel to the suspended magnet when no current was passing through the coil. The needle, when deflected to that side on which the greater deflection is obtained, will, instead of moving from a stronger to a weaker part of the magnetic field produced by the current, move at first into a stronger part, and then afterwards into a slightly weaker part. The effect of this arrangement is to make the proportional law connecting current and deflection. true for a much larger deflection from the undeflected position of the needle than if we commenced with the needle parallel to the plane of the coil for no current. But this arrangement has the disadvantage that it can only be used for currents deflecting the needle to one side of the scale, for, if the current be flowing in the opposite direction, the defect of want of proportionality between current-strength and deflection will be increased.

This plan, by means of which the proportionality on one side of the scale is sacrificed to increase that on the other, has been employed by the author, and later on by MM. Carpentier and Deprez, and others, for making proportional galvanometers.

Another device for causing the strength of the deflecting field to increase as the needle deflects is employed in the galvanometer originally devised by Mr. Mather and Professor Walmsley, and in use for many of the experiments of the first-year students at the Central Technical College. This instrument, as illustrated in Fig. 65, consists of two coils shaped as shown, and fixed so that they are separated by a distance a little less than the length of the needle. The galvanometer is placed so that when no current is passing through the coils the needle hangs symmetrically between them, and when the controlling field is a uniform one, the current