Practical Standards of Resistance.--The column of mercury as shown in [fig. 548], is the recognized standard for resistance, however, in practice, it is not convenient to compare resistances with such a piece of apparatus, and therefore secondary standards are made up and standardized with a great degree of precision. These secondary standards are made of wire. The material generally used being manganin or platinoid.

Fig. 550.--Direct deflection method of testing resistances; a useful and simple method which may be used in numerous tests. Galvanometer readings are taken through the known, and unknown resistances, and the current being proportional to the deflections, the value of the unknown resistance is easily calculated.

Resistance Measurement.--Resistance is that which offers opposition to the flow of electricity. Ohm's law shows that the strength of the current falls off in proportion as the resistance in the circuit increases. This gives a basis for measuring resistance.

There are various methods by which an unknown resistance may be measured, as by the:

  1. Direct deflection method;
  2. Method of substitution;
  3. Fall of potential method;
  4. Differential galvanometer method;
  5. Drop method;
  6. Voltmeter method;
  7. Wheatstone bridge method.

Direct Deflection Method.--This method is based on the fact that the greater the current through a galvanometer the greater the deflection of the needle; it is a simple method and is capable of extended application.

The apparatus required consists of battery, galvanometer, known resistance, and double contact key. The connections are made as in [fig. 550]. The known resistance is put in circuit with the galvanometer and after noting the deflection, the key is moved so as to cut out the known resistance and throw into circuit the unknown resistance. The deflection of the galvanometer is again noted and compared with the first deflection.

Fig. 551.--Charge and discharge key, adapted to condenser testing where the condenser is to be alternately charged and discharged. The insulated handle enables the key to be used without insulating the body.