Chapter V

it was explained that several turns of wire surrounding a compass-needle would cause the needle to move and show a deflection if a current of electricity were sent through the coil.

Such an instrument is called a _galvanoscope_ and may be used for detecting very feeble currents. A galvanoscope becomes a _galvanometer_ by providing it with a scale so that the deflection may be measured.

A galvanometer is really, in principle, an ammeter the scale of which has not been calibrated to read in amperes.

[Illustration: Fig. 110.—Simple Compass Galvanoscope.]

A very simple galvanoscope may be made by winding fifty turns of No. 36 B. & S. gauge single-silk-covered wire around an ordinary pocket compass. The compass may be set in a block of wood, and the wood provided with binding-posts so that connections are easily made.

Another variety of the same instrument is shown in Figure 111.

[Illustration: Fig. 111.—Galvanoscope.]

Wind about twenty-five turns of No. 30 B. & S. gauge cotton-covered wire around the lower end of a glass tumbler. Leave about six inches of each end free for terminals, and then, after slipping the coil from the glass, tie the wire with thread in several places so that it will not unwind. Press two sides of the coil together so as to flatten it, and then attach it to a block of wood with some hot sealing-wax.

Make a little wooden bridge as shown in Figure 111, and mount a compass-needle on it in the center. The compass-needle may be made out of a piece of spring-steel in the manner already described in