Let us first consider the influence of change of temperature on ammeter readings.
An ammeter is normally used with a shunt, i.e, a resistance connected in parallel to the instrument. The ammeter's resistance is very small, that of the shunt is even smaller; for this reason, it is usually made of copper, like the ammeter coil. The
ELECTRICITY 223
distribution of current through the instrument's coil and the shunt is determined by the relationship between their resistances. Since the relative changes of resistance caused in the copper coil and the copper shunt by changes in temperature will be the same, their relationship to each other will not change. So the readings of an ammeter connected to a shunt of the same material as the ammeter's coil will not depend on the temperature.
Now let us consider how a voltmeter's readings will be affected by temperature changes.
As you know, voltmeters must have a high internal resistance; for this the instrument's coil, which in general has not got a high resistance, should be connected in series with a high supplementary resistance made of some material with high specific resistance such as manganin or constantan. Therefore the total resistance of the voltmeter is determined basically by this series resistance and if the instrument's readings are not to vary with temperature changes, this series resistance must remain unaltered. Since the resistance of the alloys of high specific resistance mentioned above depend very little on temperature, the readings of a voltmeter are also almost independent in practice of the temperature of the surrounding medium.
Besides its effect on the magnitude of the resistances, change in temperature can also affect the elasticity of the spiral springs which hold the moving parts of the instrument in equilibrium. To obviate this effect the springs of accurate instruments are made from an alloy whose elasticity does not depend on the temperature (elinvar).