Thermo Galvanometer.—A thermo galvanometer is an instrument used to ascertain the power of a firing battery which is employed to ignite platinum wire or low tension fuzes.
The form of thermo galvanometer generally used in connection with a test table, is arranged as follows:—
Two ebonite studs, fitted with brass connecting screws, are fixed to the lid of a box containing some resistance coils, and placed in circuit with them; these studs, placed about ·3 of an inch apart, are arranged to receive a piece of platinum wire which is stretched from one stud to the other; the firing battery being placed in circuit with the platinum wire, and the resistance coils, its working power would then be tested by the fusion of the wire through a given electrical resistance, as indicated by the resistance coils put in circuit.
Another form of thermo galvanometer, which is very compact and portable, is shown at [Fig. 88]. It consists of a wooden box a, with a cover of ebonite b, within the box is placed a resistance coil c; d and e are two ebonite standards ·3" apart, the former of which is connected by a copper wire with the terminal f, the latter to the terminal g; the terminal h is similarly connected to the contact piece k, and the terminal l to the firing key m, at n; the resistance coil c is connected to the terminal g and to the copper wire n; the platinum wire (of which several lengths are used, according to the resistance of the coil c) is placed between the standards d and e. To test a battery, it is only necessary to connect it to the terminals f and h, when by pressing down the key m the power of the battery, according as to its fusing or not the platinum wires, will be ascertained; the use of the terminals g and l is to cut out the resistance, which is effected by connecting them by means of a copper wire.
Siemens's Universal Galvanometer.—Siemens's universal galvanometer is an instrument combining in itself all the arrangements necessary for the following operations:—
- 1.—For measuring electrical resistances.
- 2.—For comparing electromotive forces.
- 3.—For measuring the intensity of a current.
The instrument which is shown in elevation and plan at Pl. xxiii., [Fig. 1] and [2] respectively, consists of a sensitive galvanometer which can be turned in a horizontal plane, combined with a resistance bridge (the wire of which bridge instead of being straight is stretched round part of a circle). The galvanometer has an astatic needle, suspended by a cocoon fibre, and a flat bobbin frame wound with fine wire. The needle swings above a cardboard dial divided in degrees; as however, when using the instrument the deflection of the needle is never read off, but the needle instead always brought to zero, two ivory limiting pins are placed at about 20 degrees on each side of zero.
The galvanometer is fixed on a graduated slate disc, round which the platinum wire is stretched. Underneath the slate disc three resistance coils of the value of 10, 100, and 1000 Siemens' units are wound on a hollow wooden block, which protrudes at one side, and on the projection carries the terminals for the reception of the leading wires from the battery and unknown resistance. The adoption of three different resistance coils enables the measuring of large as well as small resistances with sufficient accuracy.
GALVANOMETERS FOR TESTING.