[Illustration: Fig. 548.--The international ohm. It is defined as the resistance of 14.452 grammes of mercury in the form of a column of uniform cross section 106.3 centimeters in length, at a temperature of 0° C. This is approximately equivalent to a column 106.3 cm. long, having a uniform cross section of 1 sq. mm. In the figure the international ohm is defined graphically. The resistance of the external circuit and the standard one volt cell is assumed to be zero.] [Illustration: Fig. 549.--Leeds and Northrup one ohm standard resistance (Reichsanstalt form); adjusted at 20° C. Low resistance standards may be properly divided into two classes: 1. those which are designed primarily as resistance standards, and 2. those designed as current carrying standards. Those of the first mentioned class are often used to measure currents up to their capacity. The above standard has both pressure and current terminals. The binding posts for the former are mounted on high posts so as to be easily accessible when the standard is immersed in oil. When used as a resistance standard of precision, it should not be subjected to a current of more than one ampere, and when used as a current carrying standard of lesser accuracy, a current of 2 or 3 amperes may be used.] 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. [Illustration: 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.