Make two small bobbins similar to those already described in connection with the volt and ammeter, but twice as long, as shown in Figure 114.
Wind each of the bobbins in the same direction with No. 36 silk-covered or cotton-covered wire, leaving about six inches free at the ends for connection to the binding-posts.
Fasten each of the bobbins to the base-board with glue. Do not nail or screw them in position, because the presence of nails or screws may impair the sensitiveness of the instrument. In mounting the bobbins, leave about one-sixteenth of an inch of space between the inside flanges, through which the needle may pass.
Connect the coils wound on the bobbins so that the end of the outside layer of the first coil is connected to the inside layer of the other coil. This arrangement is so that the current will travel through the windings in the same continuous direction, exactly the same as though the bobbin were one continuous spool.
Fig. 114.—Bobbin for Astatic Galvanometer.
Magnetize two small sewing-needles and mount them in a paper stirrup made from good, strong paper, as shown in Figure 114. Take care that the poles are reversed so that the north pole of one magnet will be on the same side of the stirrup as the south pole of the other. They may be fastened securely by a drop of shellac or melted sealing-wax.
Cut out a cardboard disk and divide it into degrees as in Figure 115. Glue the disk to the top of the bobbins. A small slot should be cut in the disk so that it will pass the lower needle.
A wooden post should be glued to the back of the base. To the top of this post is fastened an arm from which are suspended the magnetic needles.
A fine fiber for suspending the needle may be secured by unraveling a piece of embroidery silk.