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Fig. 21.—Diagrams Outlining Construction of Common Types of Current


bearing and moves with the coil to indicate the variations of current strength on a graduated scale with which the pointer registers. A steel hạir spring attached to the coil acts to restrain and control its movements and to return the pointer to zero when the current ceases to flow through the solenoid. The function of the magnetic field is to keep the solenoid steady, though as soon as an electric current passes through its equilibrium is upset and the degree of movement is proportional to the amount or pressure of the current passing through it. Many small instruments which are accurate and inexpensive have been devised for testing current strength.

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Fig. 22.—Diagram Showing Construction of Moving Coil Type Voltmeter.

For convenience the mechanism has been enclosed in standard watch movement cases in many instances.

The plunger type of indicator which is shown at C and D operates on the principle of attraction that a solenoid exerts upon materials susceptible to its influence. A curved plunger is used in that type usually intended for switch-board use. When a current is passed through the solenoid, the plunger is drawn into the interior of the coil, the amount of movement depending upon the current strength. This is indicated by a calibrated scale and

upon it.

pointer. The small battery tester which is very simple in construction works on exactly the same principle, except that the vertical plunger which is drawn into the solenoid has the scale indicated

The solenoid is kept pressed out against a stop by spring pressure which is overcome as soon as the current passes through the winding. The plunger type is not reliable for very small readings and is readily affected by any magnetic field in the vicinity.

The instrument shown at E is a magnetic vane type. In this a vane of soft iron is supported eccentrically or off center and when a current passes through the surrounding coil the

AMPERES vane is attracted toward

D the position where it will conduct the greatest number of lines of force,

GRAY & DAVIS this movement actuates the pointer attached to the vane support and a hair spring is used as in other instruments to return the pointer to zero when the current flow ceases and also to steady Fig. 23.—Typical Dash Type Amperemeter the action of the instru

Used with Modern Lighting System. ment.

The small amperemeters are used only for testing dry cells, as the scale reads only to 30 amperes. This form of instrument is also used as an indicator to show the rate of charge of a storage battery by the generator or current consumption of the lamps of the lighting system. The ordinary form of ammeter should never be used for testing storage cells and a voltmeter is necessary for this purpose. Sometimes an amperemeter is so constructed with an internal resistance that can be put in series with the solenoid coil that it will read voltage on another scale. An instrument that

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Fig. 24.-Index to Signs, Symbols and Abbreviations Used in Wiring


will indicate 30 amperes and register up to eight volts has a range that is ample for all practical purposes. Some very low reading ammeters were formerly sold extensively as coil current consumption indicators, but with the passing of the vibrator coil ignition system they are no longer used to any extent.

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