GB215986A - Electromagnetic induction micrometers - Google Patents

Abstract

215,986. Ford, J. M. June 7, 1923. Addition to 166,317. Magnetic measurements.-In the apparatus described in the parent Specification for measuring small relative movements, measurement of the relative displacement between the two elements of the normally balanced magnetic system is effected by providing a second similar normally balanced magnetic system in which the equilibrium may be disturbed by hand or otherwise by the same amount, as determined by a null reading of an indicating instrument. As shown applied to a torsionmeter, two similar laminated iron members 4, 5 are carried by one of the relatively displaceable flanges on the shaft, to the other of which is attached a laminated iron member 7. A similar magnetic system comprises two fixed members 29, 30 and a member 23 movable by a screwed spindle 25 having a scale 27 by which the movement of the member 23 can be measured. The windings 11, 12, 20, 21 on the members 4, 5, 29, 30 are connected in series with each other and with a source of alternating or intermittent direct current. Secondary windings 13, 18 are wound on the members 7, 23 and are connected in opposition and in series with an alternating-current indicating instrument 19. The primary windings 11, 12, 20, 21 are so wound that normally equal and opposite electromotive forces are induced by the secondary windings 13, 18. Displacement of the member 7 due to torsion of the shaft under test disturbs the equilibrium of the system and causes an electromotive force to be induced in the winding 13. The member 23 is then displaced by hand or otherwise until an equal and opposite electromotive force is induced in the winding 18, giving a null reading on the instrument 19. The displacement of the member 18. as measured by the scale 27, is then equal to the displacement of the member 7. The instrument mav be balanced when a propeller shaft is subjected to its normal full-load torsion, in which case the instrument 19 may indicate fluctuations in the normal torsion by defection on either side of zero. When the current in the secondary circuit is zero, the indication is independent of fluctuations in the voltage of the source supplying the primary current, which source may be the ship's lighting circuit. The alternating-current instrument 19 may be replaced by a direct-current instrument, rectifying- means being included in the secondary circuit.