f. SIR WM. THOMSON'S GRADED GALVANOMETERS. FOR MEASURING THE ELECTRO-MOTIVE FORCES AND CURRENTS IN ELECTRIC-LIGHTING OR DYNAMO CIRCUITS, OR WHEN CHARGING AND DISCHARGING SECONDARY CELLS, &C.*

The Potential Galvanometer (Fig. 20) consists of a high resistance coil C, fixed to a wooden platform P, and a magnetometer M. The coil C contains about 7000 turns, or over 2000 yards of German silver wire of No. 32 B. W.G., and has a resistance of fully 6000 ohms. The platform P supports the coil and the magnetometer. The magnetometer consists of a quadrantalshaped brass box, with a mirror bottom and glass cover, inside which a light system of magnetic needles and aluminium index or pointer is pivoted by means of a sapphire cap resting on an iridium point. The sensibility of the instrument is altered by putting the magnetometer nearer to or farther from the coil. The sensibility can be further varied by means of a semi-circular magnet placed over the magnetometer, as shown in the figure. The intensity of the field produced by this magnet is determined and painted on it, with the date of determination. This magnet renders the instrument less liable to disturbances due to other magnets, or to iron, or dynamos, in its neighbourhood. A scale is engraved on the platform P, the number at any division of which indicates the number of divisions the magnetometer needle will be deflected by one volt when it is in a magnetic field of C.G.S. unit intensity, and when the front of the magnetometer box is at that division.

To find the volts corresponding to any given deflection of the

needle -

Rule.-Multiply the number of divisions in the deflection by the intensity of the field, and divide by the number at the division exactly under the front of the magnetometer on the platform scale.

*These instruments are illustrated in Figures 20 and 21, which are engravings from photographs of the instruments.

The Current Galvanometer (Fig. 21) is, with the exception of the coil, identical in every respect with the potential galvanometer. The coil in this instrument consists of a few turns, or of a single turn, of thick copper strip, and is capable of conveying very strong currents (100 amperes in the ordinary instruments).

To find the number of amperes corresponding to any particular deflection with this instrument

Rule-Multiply the number of divisions in the deflection by the intensity of the field, and divide by the number at the division exactly under the front of the magnetometer on the platform scale.

Directions applicable to both Instruments.-These instruments should be so placed that when no current is flowing in the coil, and no magnet is near the magnetometer, the index points to zero. When the semi-circular magnet is used on the magnetometer, the intensity of the field is nearly equal to the number on the magnet plus 17 for the intensity of the earth's field in the British Islands. If the earth's field alone be used, its intensity at the place of observation must be known. It may be taken from the Chart of Horizontal Force, of the Admiralty Compass Manual.

A very convenient set of electrodes and spring contact clips, as shown in the Figures 20 and 21, is supplied along with each of these instruments, whereby the galvanometers may be quickly brought into or cut out of any electric circuit without disturbing the current in that circuit.

5 to 9

C3 C4

TABLE XX.-9. CROMPTON-BÜRGIN MACHINE. TRIALS WITH HORIZONTAL SHUNT MACHINE, AT CHELMSFORD, June 24, 1882.

Trials 1 to 4 with Type C, Machine. Bobbin, 0545 ohms. Field Magnets, 2 in series. Resistance, 10.5 ohms.

APPENDIX.

Fusion of Solids.-The following are the melting points of a few of the more important substances. The last seven are given on the authority of Daniell.

Mercury,...

Ice,

Alloy-Tin 3, lead 5,

630

773

- 38° Bismuth,..

Lead,.

Zinc,..

Silver,.

873

Sulphur,

228

Latent heat of fusion of ice, about 140 British units; of tin, 500.

Flow of Gases.— -Let the pressure, bulkiness, and absolute temperature of a gas within a vessel be P1, 1, 71, and without the vessel, P2, V2, T2; and let po vo be the value of pv for the absolute temperature of melting ice. (See page 278.) Let be the ratio in which the specific heat of the gas is greater at constant pressure than at constant volume;

Let O be the area of an orifice through which the gas escapes from the vessel;

k, a co-efficient of contraction, or of efflux, so that the effective area of the orifice is k 0;

u, the maximum velocity which the particles of the gas acquire in escaping, when there is no friction;

W, the weight of the gas which escapes in a second; then,

Values of y: air, 1·408; steam-gas, about 1·3.

Values of the co-efficient of efflux k for air, with effective pressures of from 23 to 1·1 atmosphere (Weisbach):—

Conoidal mouthpieces, of the form of the con

tracted vein,..

Circular orifices in thin plates,.

k

o'97 to 0.99

0.563 to 0.788

Outflow of Steam—Rough Approximation.-Let p1 be the internal and P2 the external absolute pressure; 9, weight of outflow per unit area per second; then when p2 = or P1, q=P1÷70 nearly; and when P2 & P1 = (P2 ÷ 42) √ {(P1 - P2) ÷ ÷ P2}, nearly. Contraction for safety valve openings about 0.6.

ADDENDUM to PART III., page 132.

Levelling by the Barometer.-To correct the difference of level given by the formula, for variations in the force of gravity, divide by the co-efficient of g1 in the note to page 245.

ADDENDUM to PART VII.

Friction of Leather Collars.-The friction of the leather collar of a hydraulic press plunger is equal to the pressure upon a ring equal in circumference to the plunger, and of a breadth which, according to Mr. William More's experiments, is about of the depth of bearing surface of the collar; and according to the experiments of Mr. Hick and Mr. Luthy, from 01 inch to 015 inch, according to the state of lubrication of the collar.

ADDENDUM to PART VIII., page 274.

Additional Resistance of Ship, due to short after-body.-Let v be the speed in knots; 7, the proper least length of after-body, in feet = 2; ', the actual length of after-body; S, the area of midship section, in square feet; sin 2, the mean of the squares of the sines of the angles of obliquity of the stream-lines of the after-body; then, additional resistance in lbs.