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resistance, causing its temperature to rise. Immediately above this “ heater ” is placed a very small thermo-couple which closes the circuit of a coil consisting of a single turn of wire suspended by a quartz fibre. A. permanent magnet produces a magnetic field through the coil.

The thermo-galvanometer (fig. 9) has practically no self-induction or capacity, and can therefore be used on a circuit of any frequency (even up to several million per second), while currents as small as 20 micro-amperes may be measured. It is equally correct on continuous or alternating currents, and can therefore be accurately standardised by continuous current, and used without error on circuits of any frequency or wave form. It is especially adapted for telephonic and wireless telegraphic measurements.

The following table gives approximate sensitivities of some thermo-galvanometers, at a scale distance of 1,000 mm.

Resistance of heater. Current to give 250 mm. deflection. Current to give 10 mm. deflection. P.D. to give 250 mm. deflection. P.D. to give 10 mm. deflection.

ohms. about 1,000 , 400 „ 100 „ 40 „ 10 „ 4 „ 1 1 microamperes. 110 175 350 550 1,100 1,750 3,500 10,000 microamperes. 22 35 70 110 220 350 700 2,000 ■ milli-volts. 110 70 35 22 11 7 3:5 10 milli-volts. 22 1 14 7 4.4 heater close to 2.2 ' junction. 1-4 0-7J r heater lowered 2:03 away from L junction.

Duddell Thermo-Ammeter. (Fig. 10.)

The principle of this instrument is the same as that of the thermo-galvanometer. The moving coil, however, is supported on pivots and fitted with a pointer.

The heater consists of a small sheet of platinised mica in instruments giving a full scale deflection with 20 milliamperes or less, and in other thermo-ammeters, of a platinum grid.

Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 98,63 %.

La langue de reconnaissance de l'OCR est l'Anglais.