Heating Error

In voltmeters the error due to alteration of resistance of the coil by heating. If too strong a current is sent through the instrument, the coils become heated and their resistance increased. They then do not pa as much current as they should for the potential difference to which they may be exposed. Their readings then will be too low. One way of avoiding the trouble is to have a key in circuit, and to pa only an instantaneous or very brief current through the instrument and thus get the reading before the coils have time to heat. The heating error does not exist for ammeters, as they are constructed to receive the entire current, and any heating "error" within their range is allowed for in the dividing of the scale.