In the former case the control is generally due to gravity, the plates being so balanced on the knife edge that they tend to take up a certain fixed position from which they are constrained when the electric forces come into play, their displacement relatively to the fixed plates being shown on a scale and thus indicating the P. D.
The magnet is in equilibrium under the influence of the couple VM due to the vertical component V, and the couple due to the fact that the centre of gravity is slightly on one side of the knife-edge.
The chief difficulty with this form of instrument is that it is very sensitive to changes of temperature, for such changes not only alter M but also in general cause the centre of gravity of the system to be displaced with reference to the knife-edge.
- This includes the case of a corn- o, pound pendulum in which the ____________ ______________ knife-edge is replaced by a cylin C drical pin.
the line j oining the end knife edges must be exactly bisected by a line drawn perpendicular to it from the fulcrum knife-edge.