This form of galvanometer, though less delicate than the preceding one, is still a very sensitive one, and should only be applied in the case of fine and delicate tests. _Differential Galvanometer._--A differential galvanometer consists of a magnetic needle surrounded by two separate coils of equal length and material carefully insulated from each other and wound in opposite directions. In using it one circuit acts against the other. If a current of equal strength were passing through each there would be no deflection of the needle, because the influence in both directions is equal. If one current were stronger than the other, the needle would be deflected by the stronger. This form of galvanometer will be found extremely useful in connection with a system of electrical tests. Latimer Clark's double shunt differential galvanometer is the instrument best adapted for submarine mine tests. _Detector Galvanometer._--A detector galvanometer is usually made with a vertical needle, and is employed to detect and roughly estimate the strength of a current where no particular accuracy is required. It consists of a magnetic needle pivoted in the centre of a coil of insulated wire, and having an index needle attached to move with it, the latter appearing on a dial, divided into 360 equal arcs or portions: a diagram of such an instrument is shown at Fig. 86. 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:--