ELECTRICAL MEASURING INSTRU MENTS. The four fundamental electrical quantities which are being constantly measured in electric circuits are ohms, amperes, volts and watts. Another quantity of much commercial importance is watt hours.
The usual method of measuring ohms, that is, the electrical resistance of a circuit, is to use a Wheatstone Bridge, which is described elsewhere. The electrical resistance of a cir cuit may also be measured by what is called gall of potential method)) which consists in sending a measured current through the cir cuit and measuring the difference of potential between the terminals of the circuit, as illus trated in Fig. 1. When the current is measured in amperes and the difference of potential is measured in volts, the resistance is obtained by talcing the ratio of the volts to the amperes. The instruments used in this measurement are ammeters and voltmeters, which are described below.
In measuring amperes, advantage may be taken of any one of three different physical effects of the electric current : (1) The elec tro-chemical effect, (2) the magnetic effect, and (3) the heating effect. Instruments which are used for measuring currents by taking advan tage of the first of these phenomena are called voltameters. When an electric current is passed through a dilute solution of sulphuric acid and water, the water is electrolyzed and the com ponent gases, oxygen and hydrogen, are given off respectively at the two metallic terminals by means of which the current enters and leaves the water. These two terminals may be placed in the two limbs of a vertical U tube, such as the arrangement illustrated in Fig. 2, where A and B are the two ends of the U tube and EE are the two metallic electrodes. The arrow shows the direction of the flow of the electric current from the battery. A riser C is provided to maintain the supply of acidulated water as the gases collect in the upper limbs of the tube.
A water voltameter such as is here illustratid has not been found convenient or satisfactory for measuring currents, and voltameters in which the electrolytes are the solutions of salts of metals are preferred for actual measurements. Copper plates in a solution of copper sulphate may be used or 'silver terminals in a solution of nitrate of silver. Voltameters are not now
used in practice, as they are not sufficiently convenient for general use; but the silver volta meter at one time proved to be so satisfactory for use as a standard that the International unit of current (the ampere) is defined as the current which flowing for one second through a suitable voltameter will deposit .001118 grams of silver on the cathode.
Most of our common instruments for meas uring currents depend upon the magnetic effect of the current for their indications, and each is really a modified galvanometer provided with a pointer to indicate the deflections of the needle or movable coil. Such instruments ar ranged for convenient, everyday measurements of electric currents are generally called am peremeters or ammeters, and they are made in numerous forms, some of them intended to be mounted upon switchboards in dynamo rooms, and others made up in more or less portable form so that they may be carried around to be used at any convenient place. The switchboard instruments — namely, those in tended to be mounted on switchboards — are used in large numbers in electric lighting plants or works, where they may be seen mounted upon marble or slate boards along with switches for controlling the current. They are there used to show the dynamo attendants how much cur rent is being generated by the plant at any moment and what proportion is furnished by each dynamo.
Portable forms of these instruments are ordinarily used in laboratories for experimental work.
According to the mechanical details entering into their construction electromagnetic ammeters may be roughly divided into three classes: (1) Those having soft iron parts which are moved by the magnetic attraction set up by the current in the coils of the instrument ; (2) those having permanently magnetized parts which are acted upon by the magnetic force set up by a current in the coils of the instrument, either the coil or the magnet moving under the influence of the magnetic force; (3) those having no iron in their construction, but having two coils, one of which is moved by the magnetic force ex erted between them when a current flows in both.