and therefore v2 is a linear sawtooth with constant frequency and amplitude v1 must therefore also be a linear sawtooth but its amplitude will vary as cm varies the sawtooth output voltage of a4 is equal to v1 v2 39 and is applied to capacitor c2 which is alternately connected to ground and to the input of integrator a5 by way of switch sw1c since c2 is grounded while v1 v2 39 is rising and connected to a5 while it is falling the current into the integrator and therefore the output voltage v3 are proportional to the magnitude of v1 v2 39 which is of course characteristic of the movement of the piston 57 it will be appreciated from the foregoing that an indicator device as described can be conveniently attached to a gyratory crusher or other apparatus and is operative to provide a voltage signal which is readily adapted to give an accurate indication of mantle position in this regard the mantle position is directly related to the position of the piston in the supporting cylinder and the probe device cooperates with that piston so as to accurately monitor its movement as the device can be contained at least in part within a cavity of the piston it requires limited space beneath the cylinder and does not prevent the piston from reaching its lowermost position in the cylinder.