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This instrument is a special form of Wheatstone’s bridge designed for use with electrical resistance pyrometers. Two small dry cells, which are self-contained in the instrument, are sufficient for its operation. To take a reading with it, a handle which controls a sliding contact is turned until a pointer indicates that the “balance point” is obtained. The temperature of the pyrometer can then be read directly off the scale. One of its chief features is the arrangement of the scale in a spiral fashion on a drum, permitting readings to be obtained to one degree, even up to 1,200° C., and rapidly varying temperatures may be followed and measured. A series of pyrometers can be read from one central station by means of an indicator and switchboard. No corrections of any kind have to be applied.

Callendar Electric Recorder.

“ Engineering” (London), Vol. 67, page 675, 1899.

Like the Whipple temperature indicator this instrument is also a special form of Wheatstone’s bridge, and is used chiefly in connection with platinum resistance pyrometers, or thermometers. The galvanometer of the “ bridge ” carries a boom, and when deflected this boom operates one or other of two relay contacts, which in turn operate electro-magnets. These electro-magnets release a clockwork mechanism, which then draws a sliding contact on the slide wire of the bridge backwards or forwards according to the direction in which the galvanometer boom has been deflected. The bridge thus automatically maintains the galvanometer in the balanced position. The sliding contact carries a pen which moves over a paper chart carried by a drum which rotates once in 25 hours. The charts can be calibrated to read directly in temperature degrees. The instrument has varied uses, and by adjusting the resistances the temperature range across the charts can be varied within wide limits. The standard chart for blast furnaces is calibrated for the range of 400—1000° C., whilst for use as a clinical recording thermometer the instrument with a range of 10° C. across the scale can be used. For this latter work the instrument is proving itself very useful. In

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