be taken to be equal to the electromotive force E. The terminals

of the galvanometer are connected only to a small portion r of R, so that the difference of potential at the ends of the galvanometer circuit is Er/R. If the resistance of the galvanometer be p and the resistance p' shewn in the figure be not inserted, the current passing through

Р

سپند

Fig. 130.

the galvanometer will be Er/pR, and the permanent deflection will be given by the equations

If the galvanometer resistance p is unknown it may be obtained, or eliminated from the above expression, if a known resistance p' is inserted in the galvanometer circuit and the steady deflection ' observed. We have in that case

and combining this with the equation obtained for the case when p' is not inserted, we find

The condition most favourable for the determination of p by this method is that in which p and p' are equal, so that the introduction of p' approximately halves the deflection.

The remarks made on page 350 as to the best way of

the present exercise the quantity λ should be independently determined.

Determine the capacity of the condenser provided, charging it by two Leclanché cells and discharging it through the galva

nometer. Take four observations of the first throw, two of which should be made with the battery reversed.

Connect the galvanometer to the two Leclanché cells as shewn in Fig. 130, adding a commutator in the battery branch. Make R about 10,000 ohms, and r such that a deflection of about 11 times the discharge throw is obtained when p'= 0. Measure the deflection with the battery current in each direction. Insert a resistance p' such that the deflection is about half what it was in the previous case, and again make two determinations of the deflection, reversing the battery current between the two observations.

Measure the time of swing and the logarithmic decrement, following the instructions given on page 352. Record your observations as follows:

3 October, 1900.

Standard Condenser marked 1/3 microfarad.

Galvanometer No. 5232.

1. Discharge of Condenser after being charged by two Leclanché Cells.

First deflection (mean of two deflections in

(a) p=0. Deflection (mean of one observation in each direction)

(b) p=6170. Deflection (do.)

Hence galvanometer resistance

A previous direct measurement of the galvanometer resistance had given p = 6570. Either value may be adopted in the subsequent calculation.

3. Determination of time of oscillation.

Time of 20 swings (average of 3 observations) = 74.9 seconds

APPENDIX.

THE following details, referring chiefly to the dimensions of the apparatus used in the Physical Laboratory of the University of Manchester, will probably prove useful to teachers.

SECTION

IV.

V.

SPIRIT LEVEL.

The ordinary 8 inch "brass adjusting level" is suitable. If the tube is not graduated, a paper scale may be gummed to it.

CALIBRATION TUBE.

A tube about 20 cms. long, of 7 cm. external and 07 cm. internal diameter, may be used. The tube and mercury should be quite dry.

VI-VIII. BALANCE AND DENSITY.

XI.

XIII.

A piece of quartz of 100-200 grams is a suitable body to weigh, since it can easily be kept clean, and therefore of constant weight. As quartz is unacted on by water it is also suitable for the density determinations.

MOMENT OF INERTIA.

The block should be suspended by means of a thin soft wire free from kinks.

COMPOUND PENDULUM.

A brass bar about a metre long, 2 cms. broad and 5 cm. thick, is suitable. The steel knifeedges are supported on glass plates.

SECTION

XIV. BEAMS.

XV.

XVI.

XVII.

Straight-grained beams about a metre long, 1.3 to 2 cms. broad and 7 to 9 cm. thick, should be used.

RIGIDITY.

The thinnest steel pianoforte wire (No. 30) and a wire about No. 25 are suitable. The upper end of each wire is soldered into a hole in the centre of a short length of brass wire of 15 to 2 mms. diameter, the lower end into a hole in a brass screw of 3 mms. diameter which screws into a hole in the axis of a cylindrical brass weight about 4 cms. diameter and 5 cms. long. The upper short wire is clamped in the support.

VISCOSITY.

The tubes should have a length of 50 to 80 cms., the bore should be 5 to 8 mm. A solution of common salt of density 1.05 to 1·10 (about 10%) is suitable.

SURFACE TENSION.

The thin glass used for covering microscope slides is suitable for the balance method.

XVIII. EXPANSION OF A SOLID.

XIX.

XX.

Tubes of about 1 cm. diameter and 60 or 70 cms. long are convenient. EXPANSION OF A Liquid.

The graduated stem of a broken 0° to 100° C. mercury thermometer serves for the stem of the dilatometer. The bulb should have about 4 times the volume the bulb of the thermometer had. PRESSURE COEFFICIENT OF A GAS.

The bore of the capillary tube is about 7 mm. and the bulb about 10 cms. diameter. The distance of the fixed mark on the capillary tube from the bulb is about 20 cms. A little strong sulphuric acid should be placed in the bulb to keep the air dry. By warming the bulb slightly after a little mercury has been poured into the open tube, the volume of air enclosed may be reduced to the desired value.