Fig. 12.—Siemens’ Thermo-electric Indicator.
In suspended coil instruments furnished with a pointer, the construction differs only in detail from the foregoing. In place of the mirror, a light pointer is attached to the suspension so as to rest on the coil and a scale is furnished over which the pointer moves. [Fig. 12] is an example of this type, made by Messrs Siemens, the suspension being contained in the tube which rises from the body of the instrument. The maximum length of scale moved over by the extremity of the pointer is about 6 inches, as a longer and therefore heavier pointer would reduce the sensitiveness below the point requisite for thermo-electric work.
In the double-pivoted type, the suspension is eliminated, and pivots are fastened to each end of the moving coil which rest in bearings. The turning of the coil is made to compress a hair spring, made of phosphor-bronze; and when the current ceases the unwinding of this spring restores the coil to its former position. The coil carries a pointer which moves over a scale. These instruments are not so sensitive as those in which the coil is suspended, but can be made sufficiently sensitive to work with any kind of junction in practical use. The pivoted form is cheaper and stronger than the suspended type, and is used whenever sufficiently sensitive.
The “Uni-pivot” galvanometer, made by R. W. Paul, is shown in [figs. 13] and [17]. The coil, which carries the pointer, is circular, and moves round a spherical core of iron placed between the poles of the magnet. A hole is drilled in the iron core, and the coil rests on a single bearing at the bottom of this hole. A phosphor-bronze control-spring serves to restore the coil to the zero position. The lessened friction due to the use of a single pivot enables this instrument to be made very sensitive when needed, so that a relatively small rise in the temperature of a junction may cause the pointer to traverse the whole length of the scale.
Fig. 13.—Principle of Uni-pivot Galvanometer.
Special Features of Indicators.—All moving-coil instruments, whether suspended or pivoted, are liable to alteration of the zero point owing to what is termed “creep.” The suspension strip, when first fixed in position, generally possesses a certain amount of initial torsion, which comes into operation gradually and causes a slight movement of the coil. Similarly, in a pivoted instrument, the strength or shape of the control-spring undergoes a gradual alteration at first, causing the pointer to move away from the zero position. For this reason adjusting arrangements are fitted by means of which the spot of light or pointer may be brought back to the zero. This creeping ceases after a time—often requiring twelve months—and if not subjected to any strain, error from this cause does not recur to any notable extent. With a mirror galvanometer it is better to move the scale, or turn the galvanometer round on its axis to restore the correct zero, rather than to twist the coil back; but with a fixed scale and pointer the only remedy is to turn the coil bodily round. In a single-pivot indicator constantly used in the author’s laboratory, the creep amounted to a movement of the end of the pointer through an angle of 2 degrees in the first few months, since when, after the lapse of several years, no further alteration has occurred. It is advisable to test the zero point of an indicator from time to time by breaking the circuit, and if an error be discovered the pointer should be re-set, or an allowance made in taking a reading.
The resistance of an indicator should be so high that the readings should not be perceptibly altered by any fluctuations in the resistance of the circuit which may arise in practice. If leads of considerable length were used to connect the pyrometer with the indicator, and were subject to fairly large alterations of temperature, the consequent changes in the resistance of such leads would be noticeable on a low-resistance indicator; and similarly, if a pyrometer were inserted at different depths in a furnace at separate times, thus heating up varying lengths of the junction wires, a discrepancy would arise for the same reason. The resistance of an indicator, however, cannot be raised beyond a certain point without reducing the sensitiveness below the required limit. A mirror galvanometer of the type described may have a resistance—partly in the coil and partly in an added series resistance—of 1000 ohms or more, and still be sufficiently sensitive; and in the latest types of instruments provided with pointers the resistance may be made as high as 1000 ohms, although it is more usually 400 to 500 ohms. Many indicators are in use, however, in which the resistance is 100 ohms or less. As, from Ohm’s law, the current varies inversely as the total resistance in the circuit, any alteration in resistance should be small relatively to the total to render the error negligible. This point is made clear in the following example:—
Example.—A thermocouple and leads have a resistance of 5 ohms and are subject to alterations amounting to 1 ohm. To find the errors resulting when indicators of resistances 800, 400, and 50 ohms respectively are used.