This instrument should be of small size and portable form, and as sensitive as it is possible to make it, under such conditions.
_Three Coil Galvanometer._--The three coil galvanometer is provided with a vertical needle, and is in other respects very similar in appearance to the detector galvanometer before described. It is formed with three coils of 2, 10, and 1000 ohms resistance; each coil is connected with a brass plate on the top of the box which encloses the whole, and may be switched into circuit by means of a plug at will. The object of the three resistances is to suit the different resistances that may occur, with a perfect, or imperfect state of the electrical combination in connection with each mine. A diagram of this instrument is shown at Fig. 87, the dotted portions are inside the case.
_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.