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ID number:  BIRRC-P0005
Institution:  Research and Cultural Collections
Named collection:  Historic Physics Collection
Artist / Maker:  The Cambridge Scientific Instrument Co. Ltd.
Title / Object name:  Double Thread Recorder
Object type:  Electrical instrument
Place made:  Cambridge
Date made:  c. 1920
Materials:  Brass, steel spring, metal drum for paper charts, black metal base in wooden case with glass door.
Measurements:  14 x 11.5 x 15.25 in. 35 x 39 x 269 cm

This is an instrument for recording two weak electrical signals (such as the emfs of thermocouples) on a paper chart. These low voltage signals have to be measured with a suspended coil galvanometer, but such a galvanometer cannot support a pen dragging on a paper chart. This ingenious ‘thread recorder’ solves this problem and permits alternate measurements from two different sources.
The suspended coil supports a counterbalanced pointer, which hangs just above the paper chart wound around a slowly rotating drum. There are two clockwork drives. The first turns the drum, and every 30 s it also releases the second drive mechanism. This mechanism causes a bar to drop onto the pointer so that it presses an inked thread momentarily against the chart making a mark. A switch then connects the galvanometer coil to the other input and 30 s later the bar drops again but onto a different coloured thread. The whole operation continues repeatedly producing two different coloured lines on the chart. The mechanism gradually advances the inked threads.

Full scale sensitivity is about 10 mV. The drum rotates once every 2 or 24 hours.

No 31406 DELETED In wooden case with glass door, lockable, and glass window on top. 3 metal feet. Sliding panel in back wall. Also a brown paper packet with spare charts. Pocket marked in handwriting '200 Charts No 2071 / Univ. of B'ham / AC 4198 / MJ'. 2 packets."

Inscriptions / Translations:  The Cambridge Scientific Instrument Co. Ltd. Cambridge, England . No. 34106

Notes:  The instrument has a hand written calibration showing ‘vapour pressure in mm of mercury’, revealing something of its last usage

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