276,080. Nalder Bros. &. Thompson, Ltd., and Lipman, C. L. May 20, 1926. Induction relays.-In an induction instrument or relay in which a conducting hollow cylinder rotates in the gap between an external electromagnet and an internal magnetic core, the internal core has polar projections symmetrically arranged with regard to the polar projections of the outer electromagnet and extending in close proximity to the internal surface of the cylinder. One or more supplementary windings are placed on the polar projections of the inner core and optionally also on those of the outer core. Fig. 1 shows a single-phase wattmeter having a potential winding V on a laminated core B, between the poles E<1>, E<2> of which is situated the conducting cylinder D. The magnetic circuit is completed by an internal cruciform core F, of which the polar projections F<3>, F<4> in line with the external poles E<1>, E<2> are unwound and provide a path of reduced reluctance for the main flux, while the polar projections F<1>, F<2> are wound with coils C<1>, C<2> in the load circuit. The coils C<1>, C<2> may be connected in series or parallel or in opposition, or one may be omitted. The cross-section of the wound poles F', F<2> is preferably considerably less than that of the unwound poles F<3>, F<4>. The potential winding V produces a flux shown by chain dotted lines, and the series windings C<1>, C<2> produce a flux shown by broken lines; these fluxes are at right-angles where they traverse the drum D. An auxiliary winding K short circuited on itself or through a resistance R may be placed at any part of the magnetic path of the potential magnet in order to produce exact quadrature between the shunt and series fluxes. For measuring the total power on an unbalanced polyphase system, two or three such elements may be mounted coaxially, the drums being fixed to a common axle. The torques due to the various elements may be equalized by magnetic shunts. For use as a voltmeter or an over or under volt relay, the winding C<1> is of fine wire connected, in series with a high resistance, across the mains. Fig. 6 shows a differential or paralleling voltmeter in which coils H<1> - - H<4> H<4> are placed on projections of the poles E', E<2> of the potential magnet and are energized through a non-inductive resistance R' from the secondary winding S of a transforming device, the primary of which is the potential coil V, connected to the source of one of the pressures to be compared. The fluxes due to the windings H<1> - - H" combine with that due to the coil V to produce a field rotating in a counterclockwise direction. The other source of voltage to be compared is connected to coils C', C<2> on the poles F', F<2> of the inner core F and produce a field which combines with that due to the coil V to produce a resultant field rotating in a clockwise direction. A torque is therefore exerted on the drum D due to the difference in voltage between the two sources. The coils V, S are preferably wound concentrically. In order to damp the motion of the drum the projections F<3>, F<4> may be extended in a plane at rightangles and the poles of a permanent magnet brought into proximity to the drum. Additional windings may be placed on the poles F<3>, F<4>. The core F may alternatively have three, six, or eight poles instead of four. The potential magnet may have more or less than four projecting poles. A voltmeter may be formed similarly to the instrument shown in Fig. 6 except that the coils C<1>, C<2> are conneted in parallel with the coils H<1> - - H<4> and the resistances R<2> in series with the coils C<1>, C<2> is of eureka or other material of negligible temperature coefficient, so that the readings of the instrument are approximately independent of temperature. Fig. 9 shows an ammeter in which the coil C<2> on the pole F<2> is connected in series with coils H<2>, H<4> on projections of the poles E<1>, E<2> and with a secondary coil S on the main electromagnet, which is excited by a winding P in the external load circuit. The coils H<2> H<4> may be omitted. The lower edge of the drum D may be shaped so as to give a uniform scale. Compensation for temperature variations may be effected in a manner similar to that described above in connection with a voltmeter. In addition, a resistance having a desired temperature coefficient may be shunted across the primary winding P. The number of supplementary coils energized by the secondary winding S may be varied by means of a switch, thereby altering the range of the instrument or relay. A reversecurrent relay may be constructed similarly to the instrument shown in Fig. 6 except that the windings C<1>, C<2> are connected in the load circuit, the winding V being connected across the mains. Normally the drum D is rotated against a limiting stop. On reversal of the phase of the current with respect to the pressure, the torque is reversed and the drum is rotated in the opposite direction when the current attains a predetermined value. Additional windings in series with the series windings C<1>, C<2> may be wound on the poles E<1>, E<2> to augment the flux due to the coils C<1>, C<2>. These additional windings may be reversed in polarity or cut out by switches to vary the range of the instrument. In a modification the coils C', C<2> are connected to the secondary coil S, the coils H<1> - - H<4> being in the load circuit. In the modification shown in Fig. 17 the current coils C<3>, C<4> are placed on one pair of polar projections and the voltage compensating coils H', H<3> on the other pair. The power-factor compensating coils K<1>, K<2> are wound on the pole F<3>, F<4> of the inner core and are short circuited through a resistance R<3>. Fig. L8 shows a reverse current relay, of which the current winding consists of a coil C<2> wound on the pole F<2> and the voltage compensating coil H<1> is wound on the opposite pole F<1>. The powerfactor compensating coils K<1>, K<2> are placed on the limbs B', B<2>. Fig. 19 shows a reversecurrent relay y having a four-polar external magnet wound with series coils L<1>, L<2> and with shunt coils N<1> N<2> in series with coils C<1>, C<2> on the internal core. In a modification, Fig. 20, current coils I', I<2> are wound on the poles E', E<2>, the coils H<1> - - H<4> are wound on the polar projections on the poles E', E<2> and are energized from the secondary coil S, and windings C<3>, C<4> on the internal poles F<3>, F<4> are con. nected to an independent source of alternating or direct current and serve to act with or oppose the flux due to the main winding V so as to vary the sensitiveness of the relay. Fig. 21 shows an impedance relay in which the main magnetising coil P is the series coil. A coil C<1> on the pole F<1> is energized from the secondary winding S. The potential coil V is wound on the opposite pole F<2>. Under normal conditions the torque due to the coils P, C<1> is less than and opposite in direction to that due to the coils P, V and the relay is inoperative. On the occurrence of a fault the torque due to the current coil exceeds that due to the potential coil and the relay operates. The moving member is preferably fitted with an inverse time delay device. Fig. 22 shows a reverse power relay in which the current coil C<2> is wound on the vertical pole F<2> of the internal core and the voltage coils are wound on horizontal poles J<3>, J<4> of the electromagnet. Power-factor compensating coils K<1>, K<2>, shortcircuited through a resistance R', are wound on the poles F<3>, F<4>. Fig. 23 shows a differential relay of the balanced-current type. The main electromagnet carries duplicate primary coils P<1> P<2> and a common secondary coil S which energizes a coil C' on the pole F<1> of the inner core. The primary coils P<1>, P<2> normally neutralize one another, but on the occurrence of a fault the effect of one coil is in excess of the other and causes operation of the relay. According to one of the Provisional Specifications, the invention is applicable to frequency meters.