switch 61 as shown in fig3 is in an on position and as such supplies through conduits 62 and 63 a dc current to coil 64 which represents the electromagnetic coil of the magnetic clamping assembly 2 illustrated in fig1 and 2 a diode 65 may be connected in parallel with the coil 64 to suppress or filter any current spikes or surges which may be present the switch 61 is movable to a momentary third position engaging terminals 66 and 67 by which the polarity of coil 64 is temporarily reversed a resistor 68 may be provided for reducing the power or force of the electromagnetic field created by the coil 64 in the momentary third position the purpose of the momentary third position will be more fully understood hereafter in operation the testing apparatus t may be placed on the collar c as shown in fig1 and 2 so that the pole shoes 21 and 22 rest thereon the electric circuit box 26 may be connected to an ac power source and the switch 61 placed in the 34 on 34 position of fig3 thus an electromagnetic force is created at the pole shoes 23 and 24 attracting the entire tester apparatus t to the coupling c and holding it firmly in place the studs 33 and 34 are adjusted by turning the large diameter portions 35 and 36 thereof by hand.