by
COLE & MORGAN, Inc.
CHAPTER VI. ELECTRIC MEASURING INSTRUMENTS
Galvanometers, Ammeters, Voltmeters. How to Make a Galvanometer.
If several turns of wire are wrapped about a compass, and a current of electricity sent through the coil, the compass needle will be deflected. Such an instrument is called a *galvanometer* and may be used for detecting very feeble currents.
Figure 67 shows a very simple form of galvanometer which may be made by winding about fifty turns of No. 36 B. & S. Gauge single silk covered wire around an ordinary pocket compass. The compass is set on a block of wood and the wire wound around both the compass and the block. The terminals of the wire are connected to two binding posts.
[Illustration: FIG. 67.—A Simple Galvanometer.]
A galvanoscope of this sort may also be used for a polarity indicator to distinguish between positive and negative poles. Connect an ordinary dry cell to the galvanometer and the needle will immediately swing to one side or the other, depending upon which way the current flows. The carbon of the dry cell is *positive* and the zinc is *negative*. Note which way the needle swings, whether toward the binding post connected to the positive pole or the one connected to the negative pole. It will swing towards the corresponding pole when any other source of current is connected to it and by knowing whether it points towards positive or negative it is very easy to determine the polarity.
The Construction of Ammeters and Voltmeters.
The experimenter will find a voltmeter and an ammeter to be very useful instruments about the workshop and laboratory. A *voltmeter* is an instrument designed to measure electro-motive force or electrical pressure. An instrument designed to measure rate of flow of current is called an *ammeter*.