_Daniell Signalling Battery._--At Fig. 75 is shown the manner of arranging a Daniell cell. A glass or porcelain vessel _a_ contains a saturated solution of sulphate of copper, in which is immersed a copper cylinder _b_ open at both ends and perforated by holes; at the upper part of this cylinder there is an annular shelf _d_, also perforated by holes, and below the level of the liquid; this is for the purpose of supporting crystals of sulphate of copper for the replacing of that decomposed as the electrical action proceeds. Inside the cylinder _b_ is a thin porous vessel _c_ of unglazed earthenware; this contains either water, or a solution of common salt, or dilute sulphuric acid, in which is placed the cylinder of amalgamated zinc _e_. Two strips of copper _p_ and _n_, fixed by binding screws to the copper and to the zinc, serve for connecting the elements in series, or otherwise. For the purposes of testing, either the Leclanché or Daniell battery specially arranged, or the Menotti battery, which is really a modification of the Daniell, may be used. [Illustration: FIRING BATTERIES, TESTING BATTERIES. PLATE XIX] _Description of a Menotti Cell._--A Menotti cell, shown at Fig. 76, consists of a copper cup containing some crystals of sulphate of copper and covered with a fearnought diaphragm _a_, placed at the bottom of an ebonite cell _b_; over this cup is put some sawdust, and resting on top of this is a disc of zinc _c_ on another piece of fearnought. The upper portion of the zinc and its connection with the insulated wire are carefully insulated. Fresh water poured on the sawdust renders the battery active. _Description of a Menotti Test Battery._--Fig. 77 represents a plan of the top of such a test battery with a 20-ohm galvanometer attached thereto. The connections are made as follows:-- One of the wires _w_ of the object to be tested is attached to the terminal _f_, which is also connected by an insulated wire to the copper cup _a_; the other main wire _w__{1} is attached to the terminal _g_ of the galvanometer; _h_, the other terminal of the galvanometer, is connected by a short piece of wire _k_ to the terminal _l_ of the contact key _m_; and the contact point _n_ is in connection with the zinc plate _c_; thus the current from the battery flows along the wire _w_ through the object to be tested, back along the wire _w__{1}, through the coils of the galvanometer, along the wire _k_ to the contact key _m_, and if this is pressed down to the zinc plate _c_, so completing the circuit. To steady the needle of the galvanometer a bar magnet is used, which is inserted in the space _r_. The whole of the apparatus is enclosed in a leathern case fitted with a cover and strap. This is a very compact and simple form of test battery, and will be found extremely useful in boats, &c., when placing mines in position.