AIVIMETER, or AMPEREMETER (am pere + Gk. pirpov, metron, measure). An in strument which is used to measure the intensity of an electric current, and which indicates this quantity directly in amperes (q.v.). Ammeters are constructed in numerous forms, which are based for the most part on the galvanometer (q.v.), on the intensity of attraction for soft iron exerted by a hollow coil of wire carry ing a Current, or on the electrodynamometer. As the galvanometer is used to detect and measure minute currents, so the ammeter is employed in testing and engineering to indicate large currents, and to enable an observer to read directly in amperes the current flowing at any instant in a circuit. The best form of ammeter is the Weston instrument, made in the United States, and used all over the world. It consists of a voltmeter (q.v.) or portable galvanometer, whose movable coil is connected in parallel with a low resistance formed by one or more copper wires. As the current in a circuit depends upon the fall in potential across a constant resistance (in this case the copper wire), the operation of the instrument will readily be seen.
Numerous other forms of ammeters are con• structed, the simplest of which consist of a toil of wire through which the current passes, in closing a soft iron core suspended by a spring. The amount that this core is attracted is indi cated by a pointer on a scale, which can be made regular by constructing the of suitable shape. In other ammeters a magnetic needle is placed between the poles of a strong perma nent magnet, and is surrounded by coils through which the current passes. This current in passing deflects the needle by an amount depending upon its intensity. The dynamometer or some modification of it, is often used to measure alternating currents, and consists of two coils, one of which is free to revorve against the action of a spring. When the current passes through the two coils, which are normally at right angles, there is a tendency for the movable coil to take a position parallel to the other, and the amount of motion depends upon the intensity of the current.