250 C~;RI~~NT TOPICS. [J. 1;. 1.

The Quadrature Oscillograph.-_IPoz~er Plant Engineering, Vol. XXXIX, No. 7.) JESSE B. SHERMAN, in a paper in the Proceedings of the Institute of Radio Engineers, describes this apparatus which overcomes the difficulty in the use of the conven- tional mechanical oscillograph with variable speed mirror for obser- vation of phenomena not of constant frequency. A mechanical galvanometer giving quadrature deflections in addition, provides means for producing Lissajous figures, hysteresis and other loss loops, and other two coordinate figures. This is in effect a low frequency substitute for the cathode ray tube.

The direct projection method of the Einthoven galvanometer is employed, using longitudinally bored poles and optical magnification by means of a projecting microscope. However, instead of project- ing the shadow image of a vibrating string, a slitted mask mounted on parallel strings permits restricted passage of light. Two slitted masks, one on each vibrator with their planes parallel to each other and with their slits at right angles will produce on the screen a square spot of light resulting from the intersection of the two slits. If either vibrator alone moves under the application of an alternating E.M.F. a bright line will be traced on the screen. The movement of both vibrators will result in the production of Lissajous figures precisely as in other cases of quadrature vibrations.

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