To detect what is called an electric current from a cell, you must have a galvanometer, a simple form of which we will now give you directions for making.

First plane up a piece of wood 4 inches square and ¾ inch thick; mahogany will be best for this. Now cut a circular piece of writing-paper or cardboard 3 inches in diameter. Take a pair of compasses with a pen at one leg, and describe a circle in ink just within the edge of the card, and an inner circle about ¼ inch from it. Carefully divide the circle into four quarters, and these quarters into nine equal parts, and each two sections into ten equal parts or degrees, marked °, as shown in Fig. 141. Now make a small hole in the centre of the wood square, and cub off a small needle, long enough to stand point upwards, ½ inch above the board. Make the needle fast by sealing-wax, and let it be perfectly upright. Fasten the divided circular card flat to the board with marked side upwards, and the 0 (zero) point towards the central point of one of its sides. This you had better secure by drawing two lines at right angles across the board, dividing the wooden stand into four equal squares, and let the pin pass through the centre of the card. Both the centres of the wood and card will then coincide. Now take a few feet of cotton-covered copper wire, and make them into a flattened coil, as shown in Fig. 142, leaving the two ends free. Keep the coils together, either by binding them neatly with some silk thread, or by glueing a piece of pink tape round it. Then fix it to the board in a straight line across it, so that one end points to the 0 (zero) on the card, and the other to its exact opposite point. Bore two holes on each side of the coil, and draw through them another loop of wire which will hold the coil upright but firmly to the board. Arrange it so that as many strands of wire are one side as on the other of the upright needle. Now draw the ends of the coil, and fasten one down to one corner and one to another by means of a binding-screw, taking care that the screw grasps a portion of wire from which you have scraped the cotton. Now you must have a small magnetic needle to swing within the coil, and your galvanometer is complete, as in Fig. 143. You can make this magnetic needle for yourself, if you like to take a small piece of watch-spring; soften it by heating it in the fir ; then hammer it out Bat; cut it off and shape it. Indent a hollow in its centre by a round pointed nail and hammer. Then harden it again by heating to about dull redness, and plunge it into cold water; magnetize it by drawing it several times across the poles of a strong magnet, taking care that you do this always in the same direction.

Fig. 141. - Circular Card for Galvanometer.

Fig. 142. - Insulated Coil for Galvanometer.

Fig. 143. - Galvanometer complete.