[Illustration: Figs. 529 and 530.--Galvanometer lamp and scale for individual use. The scale is etched on a ground glass strip 6 centimeters wide by 60 centimeters long with long centimeter divisions and short millimeter divisions the entire length, reading both ways from zero in the center. It is mounted in an adjustable wooden frame. A straight filament lamp (110 volts) is enclosed in a metal hood japanned black to cut out all reflected light. This form of filament makes a single brilliant line on the scale, enabling closer readings than the "spot of light" arrangement. The lamp hood is adjustable to any desired height on the support rod.] D'Arsonval Galvanometer.--This instrument has a movable coil in place of a needle, and its operation depends upon the principle that if a flat coil of wire be suspended with its axis perpendicular to a strong magnetic field, it will be deflected whenever a current of electricity passes through it. Ques. Describe the construction of a D'Arsonval galvanometer. Ans. The essential features are shown in figs. 532 and 533. The coil, which is rectangular in section is wound upon a copper form, and suspended between a permanent magnet by fine wires to the points A and B. The magnet has its poles at N and S. It is a soft iron cylinder fixed between the poles in order to intensify the magnetic field across the air gaps in which the coil moves. [Illustration: Fig. 531.--Queen reading telescope. This arrangement is utilized to measure the deflections of a galvanometer having suspended mirror moving system. It consists of a reading telescope mounted as illustrated with a millimeter scale, having a length of 50 centimeters. In use, the image of the scale is seen in the galvanometer mirror through the telescope. The eye piece of the telescope has a cross hair which acts as a reference line so that by noting the particular division on the scale when the galvanometer is at rest, the amount of deflection can be readily observed when the galvanometer is deflected. The instrument has all the necessary adjustments to set it up quickly and for bringing the cross hair and scale in focus. It is generally placed at a distance of one meter from the galvanometer mirror.] Ques. Explain its operation. Ans. An enlarged horizontal cross section of the galvanometer on line XY is shown in fig. 533. The current is flowing in the coil as in fig. 532, up on the left side and down on the right. The position of the coil when no current is flowing is indicated by _n' s'_. By applying the law of mutual action between magnetic poles, it is seen that when the current is applied, the poles developed at _n' s'_ will move into the position _n'' s''_. See fig. 119. Ques. How is the coil affected by a change in the direction of the current? Ans. The polarity of the coil is reversed and consequently the direction of the deflection.