Page:Electricity (1912) Kapp.djvu/158

ism, an instrument of this kind could be used as a standard by which to calibrate, that is, mark the scale of commercial amperemeters, but in our electric age of tramways running in all directions, dynamos working in almost every building, and with steel joists used in the construction of buildings, the value of H in any given place is an uncertain and changing quantity. The tangent galvanometer can therefore not be considered as an absolute standard for current measurements, as it was considered by scientists two generations ago; but it may still be used for the measurement of very feeble currents if proper precautions be used. Since we must standardise the instrument in any case, there is no need to make the coil enormously large in comparison with the size of the needle; we can make both coil and needle very small, and use a great number of fine wire turns in the coil. By these means it is possible to produce an instrument which is capable of measuring the one-millionth part of an ampere or even less. It is obvious that with so delicate an instrument no material pointer to indicate the deflection can be used. The pointer is in fact weightless, being formed by a beam of light reflected from a little