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*. Ammeters and voltmeters are really, in principle, galvanometers, the scales of which have been calibrated to read in amperes or volts, as the case may be. The little meters described below are very simple but quite sensitive. [Illustration: FIG. 68.—Details of the Bobbin.] The wooden bobbin which holds the wire is shown in Figure 68. The exact dimensions are best understood from the illustration. The wood can be easily secured from an old cigar box. In laying out the work, scratch the lines on the wood with the point of a darning needle. Pencil lines are too thick to permit of accuracy in small work. The bobbin should be perfectly square and true when finished. Finish by rubbing with fine sandpaper and then give it a coat of shellac. Two bobbins will be required, one for the voltmeter and one for the ammeter. Do not use any nails in putting the bobbins together. Use strong glue only. The bobbin for the ammeter should be wound full of No. 14 B. & S. Gauge double cotton covered magnet wire. The voltmeter requires much smaller wire. No. 37 B. & S. Gauge single silk covered wire will serve the purpose satisfactorily, but a finer size such as No. 38 or 40, is better. Such fine sizes will probably, however, prove more difficult for the experimenter to obtain. Sufficient wire should be used to fill the bobbin up. The wire should be wound on carefully in smooth even layers. A small hole should be bored in one of the flanges, through which to pass the end of the wire when starting the first layer. About six inches of wire should be left at both ends of the coils to make connection to the terminals with. The whole winding should be given a coat of shellac when finished. [Illustration: FIG. 69.—Details of the Armature, Bearings and Pointer.]