Hot-wire ammeters are open to the following objections: The scale divisions for equal increments of current are not equal in length, being generally much closer together in the lower parts of the scale.
In ammeters for small currents it is customary to pass the whole current through the heating wire.
Hot-wire ammeters are, however, liable to a shift of zero, and means are always provided by some adjusting screw for slightly altering the sag of the wire and so adjusting the index needle to the zero of the scale.
From this it follows that hot-wire ammeters are generally not capable of giving visible indications below a certain minimum current for each instrument.
- Another large class of ammeters depend for their action upon the fact that an electric current create; an electric field round its conductor, which varies in strength from point to point, but is otherwise proportional to the current.