Ques. For what use is the astatic galvanometer adapted?
Ans. For the detection of small currents.
It is used in the "nil" or zero methods, in which the current between the points to which the galvanometer is connected is reduced to zero.
Fig. 513.--Queen reflecting astatic galvanometer. It is mounted on a mahogany base with levelling screws. A plain mirror is attached above the upper needle. The entire combination of mirror and needles is suspended by unspun silk from the interior of a brass tube, which also carries a weak controlling magnet. A dial 4 inches in diameter and graduated in degrees, enables the deflections of the needle to be accurately read. The mirror can be used with a reading telescope and scale, or by means of a lantern, the image of a slit may be reflected from the mirror to a screen. Resistance, .5 to 1,000 ohms.
Ques. Upon what does the movement of the needles depend?
Ans. Upon the combined effect of the magnetic attraction of the current which tends to deflect the needles, and the torsion in the suspension fibre which tends to keep the needle at the zero position.
Ques. Does the astatic galvanometer give correct readings for different values of the current?
Ans. When the deflections are small (that is, less than 10° or 15°), they are very nearly proportional to the strength of the currents that produce them.
Thus, if a current produce a deflection of 6° it is known to be approximately three times as strong as a current which only turns the needle through 2°. But this approximate proportion ceases to be true if the deflection be more than 15° or 20°.