[Illustration: FIG. 2,518.--Keystone dynamometer movement. Since the law governing this type of instrument is the law of current squares, it follows that in the case of voltmeters, equally divided scales cannot be obtained. In the case of ~wattmeters~, the scale is approximately equally divided, due to the fact that the movement of the moving coil is proportional to the product of the current in the fixed and moving coils. ~The moving parts~ have been made as light in weight as is consistent with mechanical strength, and the entire moving system is supported on jeweled bearings. The motion of the pointer is rendered aperiodic by the use of an aluminum air vane moving in a partially enclosed air chamber. This method of damping the oscillations of the moving parts renders unnecessary the use of mechanical brakes or other frictional devices, which tend to impair the accuracy of the instrument. The illustration shows a ~voltmeter~, which, however, differs but little from a wattmeter. In the case of a wattmeter the fixed coils are connected in series with the line, either directly or through a current transformer, while the moving coil is connected in shunt to the line.] The angle through which the tortion head was turned, being proportional to the square root of the angle of tortion, the current strength in amperes is equal to the square root of the angle of tortion _multiplied by a calculated constant_, furnished by the maker of the instrument.
~Ques. How is the dynamometer arranged to measure watts?~
Ans. When measuring watts, the instrument should be so arranged that one coil carries the main current, and the other a small current which is proportional to the pressure.
[Illustration: FIG. 2,519.--Leeds and Northrup ~electro-dynamometer~. _It is a reliable instrument for the measurement of alternating currents of commercial frequencies_. ~When wound with fine wire~ and used in connection with properly wound resistances, it is equally useful for measuring alternating pressures, and may thus be employed to calibrate alternating current voltmeters as well as ammeters. ~To give accurate results~ the instruments must be carefully constructed and designed with a view to avoiding the eddy currents always set up by alternating currents in masses of metal near, or in the circuits. ~The constant of a dynamometer~ may be obtained with a potentiometer, but this is usually done with precision by the manufacturer and a certificate giving the value of the constant is furnished with the instrument. ~The size and cost~ of dynamometers rapidly increase with the maximum currents which they are designed to carry, and when more than 500 amperes are to be measured, the use of other instruments and methods is recommended.]