The magneto will actuate this armature vigorously as a sounder, or rattler, through about 1000 ohms resistance. It may be used for testing the continuity of torpedo and other circuits, or for testing the insulation of the permanent and other leading-wires. =For Testing Continuity.=—The poles of the magneto are connected with the ends of the circuit to be tested and the crank turned. If the armature rattles it indicates a continuous circuit. The failure of the armature to rattle will show a break in the circuit. =For Testing the Insulation of the Permanent Wires.=—Connect one pole of the magneto with the wire to be tested and the other pole to earth; or, if a cross with some other wire is suspected, the other pole is connected with that wire. If the armature rattles vigorously when the crank is turned, a leak of less than about 1000 ohms resistance is indicated; if not, the insulation resistance of the wire is about 1000 ohms. =To Test the Insulation of a Leading-Wire.=—Attach one end of it to one pole of the magneto, the other pole of which is connected by a short length of wire to an earth-plate placed in a tub of sea-water. Keeping the two ends of the wire to be tested out and dry, pay it into the tub gradually, turning the crank of the magneto meanwhile. Should there be a fault in the insulation, its existance and locality will be indicated by rattling of the armature when it reaches the water. FARMER‘S DYNAMO-ELECTRIC MACHINE, PATTERN A, AND FIRING-KEY. =Plate XVI.= For a full description of the electric machine, see “A Lecture on Galvanic Batteries, Part III” published by the Bureau of Ordnance, 1875. In general, pattern A may be considered as having an electro-motive force of sixteen to eighteen volts and a resistance of five ohms and to be capable of firing from twenty to twenty-five detonators arranged in series, or five to six arranged in as many branch circuits, or a single detonator through 1½ miles of cable such as is now issued, or through twenty ohms resistance. It is unnecessary to give more than three or four turns of the crank in order to generate sufficient current to fire; but these revolutions must be with the sun and continuous up to and including the moment of firing. In general, as more work is required from the machine, greater speed and longer time will be necessary to get the machine up to its maximum power; this time, however, is very limited and the rapid turning of the crank for half a minute may be considered sufficient. With a single detonator in circuit and a moderate amount of leading-wire, one-quarter of a turn of the crank will usually be sufficient to fire. =Testing the Machine.=—To test the machine, connect the binding-screws by a piece of metal, ship the crank and turn it with the sun. If it turn hard the machine is in good order; if it turn as easily as before the binding-screws were connected the machine is out of order.