Page:Electricity (1912) Kapp.djvu/137

Rh difficulty of obtaining a glass tube of absolutely even bore, but the further difficulty that the specific resistance of mercury, as of all metals, varies slightly with the degree of chemical purity in which the metal can be obtained, so that the so-called "mercury standard" has been discarded in favour of standards made of platinum.

Such standards are deposited in State Laboratories or Museums, and only serve as reference standards, such as the yard or the pound. For practical use other so-called secondary standards are made of some less expensive material, generally some alloy, such as German silver, manganin, platinoid, eureka metal, etc. These alloys have the advantage that their resistance is very little influenced by change in temperature, whereas copper increases its resistance sensibly when heated. For every degree centigrade of temperature rise above 15° C., the resistance of a copper conductor rises by about 0.38 per cent. All machines when at work become heated to a certain extent, since some of the energy which is passing through the machine is necessarily lost in the process of conversion from one form to another form. This lost energy is converted into heat, and thus the temperature of