936,152. Electric measuring instruments. PHILIPS ELECTRICAL INDUSTRIES Ltd. April 20, 1960 [April 23, 1959], No. 13835/60. Class 37. [Also in Group XXXVIII] A mirror galvanometer operates on the principle of keeping the light spot stationary and moving the scale and provides means for projecting an image of that part of the scale giving the correct indication of the quantity measured on to the same screen as the light spot, so that the light spot will lie on that point of the scale indicating the measured quantity. In the embodiment shown in Fig. 1, two mirrors S 1 , S 2 are carried, at a fixed angle to one another, on the moving shaft of the galvanometer coil Sp, and a curved mirror So has a scale provided on it. The arrangement is as follows: light from a source G passes through lenses L 1 , L 2 , is reflected off mirrors S 1 So and S 2 and through lens L 3 on to a frosted glass plate P, so that when coil Sp is deflected by a current passing through it, the light spot from source G will remain stationary, due to the double reflection off the mirrors S 1 , S 2 ; but that point of the scale projected as an image on to plate P will vary in accordance with the deflection of the shaft and therefore with the current in coil Sp. In an alternative arrangement (Fig. 2), only one mirror S 5 is provided on the shaft, and the light from G passes through a lens L 1 , a semitransparent mirror S 3 , through lens L 2 on to mirror S 5 , on to curved mirror So and back from So to S 5 , through lens L 2 , on to mirror S 3 and on to the plate P. In yet another embodiment (Fig. 3, not shown) two mirrors are again provided on the galvanometer shaft, and two curved mirrors, one having half the radius of curvature of the other and having the scale provided on the outside, convex, surface thereof, this mirror being arranged concentrically with the other mirror. The device may be used as a digital measuring instrument by the provision of transparent lines, separated by light screening parts, on the surface P, the distances of the lines differing from the scale lines, so that when the system is deflected, a pair of photo-electric elements at right angles arranged behind P will produce pulse sequencies having a phase difference which is fed to a counter, the count thereon varying with the phase. Alternatively, the distances between the scale lines of surface P and scale on So may be equal but the lines at an angle to each other, so that a moire pattern is produced behind the surface P. The zones move during movement of the coil system in the direction of the line, the direction of movement depending on the sense of rotation of the moving coil. The photo-electric elements must be relatively displaced in the direction of the lines.