The Leeds-Northrup Recorder.—In the Callendar recorder the boom which completes the electric circuits is pressed against the contact-surface merely by the small force due to the axial twist of the galvanometer coil, which necessitates the use of delicate mechanism if certainty of action is to be secured. A surer contact is secured in the instrument made by the Leeds-Northrup Company of Philadelphia, by means of an intermittent action which will be understood from the annexed drawing ([fig. 40]). The boom from the galvanometer terminates in a platinum tip, P, which moves between two blocks, the upper of which consists of two pieces of silver, A and B, separated by a strip of ivory, I, whilst the lower block, C, is another piece of silver, which is moved periodically up and down by an electro-magnetic contrivance not shown in the drawing. When the galvanometer is at the position of balance, the tip of the boom is beneath the ivory piece I; and when C ascends the tip P is then squeezed on to the ivory, and no current will then pass from the battery through either of the circuits E or F. If, however, the point of the boom be beneath A, owing to an alteration in the temperature of the pyrometer, then on C rising the circuit through E will be completed; and, similarly, if beneath B the circuit through F will be established. The result in either case is to bring into action a mechanism which moves a slider, carrying a pen, over a resistance wire opposed to the pyrometer in such a manner as to restore the balance. Certainty of contact is thus secured, which enables all the parts to be strongly made. The actual recorder is shown in [fig. 41], in which it will be seen that the slider carries an ordinary stylographic pen in contact with the chart. This recorder is worked on the differential galvanometer method; and the adjusting resistance, over which the slider moves, consists of a manganin wire wound on a tapered core, such that horizontal movements represent equal changes of temperature, and not of resistance, thus obviating the necessity of translating platinum-scale readings into ordinary degrees. Concordant and accurate results, coupled with robust construction, are claimed for this instrument by the makers. The other type of recorder made by this firm ([fig. 26]) may also be used in conjunction with a resistance pyrometer. In this case the movements described introduce or cut out resistance opposed to the pyrometer in a Wheatstone bridge circuit, until the balance is restored.
Fig. 40.—Principle of Leeds-Northrup Recorder.
Fig. 41.—Leeds-Northrup Recorder.
Paul’s Recorder.—This instrument, as used for thermo-electric pyrometers, has already been described. By replacing the galvanometer by a Harris indicator, and using a suitable chart, the same mechanism serves to record the indications of a resistance pyrometer.