This force is balanced by twisting the spring by means of the thumbscrew. The amount of twist is shown by the pointer B, and it is pro portional to the force exerted by the coils on each other, which in turn is proportional to the square of the current flowing in the circuit. The pointer S indicates when the movable coil is at its zero position.
Very accurate and permanent standard in struments have beep designed for measuring currents by this direct magnetic action, but they have not been made sufficiently portable to bring them into much use. The most important of these are the current balances of Lord Kel vin, one of which is illustrated in Fig. 6. The fixed and movable coils in these Kelvin balances are parallel to each other and horizontal. The force with which the coils tend to move with respect to each other when a current flows in them is directly balanced and weighed by means of a slider moving on a scale beam. In order to avoid any disturbing effect from the earth's magnetism, coils are placed at both ends of the balance arm.
Instruments utilizing the heating effect of the current are usually called °hot-wire)) in struments. If the heated wire is carefully en closed so that its temperature is not affected by air currents, it will rise a definite number of degrees in temperature for each current that is passed through it and this rise is approxi mately proportional to the square of the cur rent. This heating of the wire is indicated by its expansion in length; and the wire, if prop erly selected and protected, will take up a cor responding length with each current which may flow through it, so that measuring its length is equivalent to measuring the square of the current. A simple model of an amperemeter depending upon this action is illustrated in Fig. 7. The long, thin wire is clasped at one end in a stationary binding post and the other end is wrapped around and fastened to a small wheel of metal. This wheel is supported on steel pivots, one of which is connected to another binding post. The wire is kept under a constant strain by means of a spring which is fastened to the periphery of the wheel. When the wire is heated and thereby lengthened, the wheel is turned by the contraction of the spring and a pointer which moves over a graduated scale indicates the amount of expansion of the wire. When the wire cools again and con tracts, the wheel is pulled back into its old position by the shortening of the wire.
A refined instrument of this type suitable for reasonably accurate measurements, is illus trated in Fig. 8. The extension of the measur ing wire A B is indicated by a pointer moved by mechanism attached to the pulley D. The case
protects the wire from air currents and the use of two stretched wires, A B and B C, through only one of which the current to be measured flows, neutralizes the effect of the general tem perature of the surrounding air.
Instruments of this type are particularly useful in measuring the high frequency cur rents of radio telegraphy.
Instruments for measuring electrical pres sures, in volts, may be made of the same forms as the ammeters utilizing the magnetic effects and the heating effects of electric currents. Such instruments are ordinarily called volt meters and when they are constructed to utilize the aforenamed effects of the electric current they are really ammeters wound with coils of high resistance so that very little current will be wasted in the process of making the meas urements of the voltage. Such an instrument really measures the very small current that is caused to flow through the resistance of its winding by the voltage to be measured. The resistance of the instrument is of constant value and the voltage is therefore directly propor tional to the flow of current through the in strument. It is consequently possible to grad uate the scale so that the position of the pointer indicates volts.
In most ammeters and voltmeters the scales are so divided and marked that the divisions read directly in amperes and volts. These in struments are generally called °direct instruments.
Currents which rapidly alternate in direc tion cannot be measured by magnetic instru ments like the Weston instruments having per manent magnets; but they can be measured by instruments having soft iron parts which are moved by the magnetic attraction set up by the current in the coils of the instrument, or by instruments of the electrodynamometer class .or by hot wire instruments. In the first class of instruments, the soft iron core is always at tracted by the coil in which current flows, with out regard to the direction of the current and the attraction in an electrodynamometer is also independent of the direction of the current be cause the current reverses at the same time in both coils. Any iron cores which are used in instruments designed to measure these alter nating currents must be built up from thin strips or fine iron wires so that induced eddy currents shall not be set up in them by the reversals of the magnetism. The working parts of an instrument which operates by the attrac tion of the coil D upon a thin iron strip C, are illustrated in Fig. 9, the exterior of the case having been removed so that the working parts shall be exposed.