The range of the instrument may be changed by hanging different weights upon the needle. By increasing the number of blades the instrument can be made to measure as low as 30 volts. The form having two stationary blades and one movable blade is suitable for measuring from 200 to 20,000 volts. The quadrant electrometer or laboratory form will measure a fraction of a volt. [Illustration: Fig. 640.--Thompson astatic instrument without cover. When current passes through the coils of the moving element, the lines of force parallel to the shaft produce a torque which tends to turn the shaft and cause the needle to travel across the scale. This action is, of course, opposed by the magnetic field at right angles to the shaft acting on the two pieces of magnetic metal. These astatic instruments have no controlling springs. The two small silver spirals which conduct the current to and from the armature are made of untempered silver and exert no force as springs. The actuating and restraining forces are dependent upon the same electromagnets. The damping effect in these instruments is produced by an aluminum disc moving in a magnetic field, and is proportional to the square of the magnet strength.] Ques. Explain the construction and principle of the Thompson astatic instruments. Ans. The fields of these instruments are electromagnets wound for any specified voltage and provided with binding posts separate from the current posts of the instrument. The moving coils are mounted upon an aluminum disc and are located in a magnetic field which is parallel to the shaft and astatically arranged. Two small pieces of magnetic metal are rigidly mounted on the shaft and the astatic components of the magnetic field, which are perpendicular to the shaft, tend to keep the pieces of magnetic metal in their initial positions. When current passes through the coils of the moving element, the lines of force parallel to the shaft produce a torque which tends to turn the shaft and cause the needle to travel across the scale. This action is, of course, opposed by the magnetic field at right angles to the shaft acting on the two pieces of magnetic metal. There are thus no restraining springs, current being conveyed to the moving coil by torsionless spirals of silver wire. Thompson astatic instruments can be provided with polarity indicators, a red disc showing on the scale card where the poles are reversed. The effect of external fields is eliminated by the astatic arrangement of the fields and the moving parts. A field which tends to increase the torque on one side of the armature diminishes it to a corresponding degree on the other side. The damping effect in these instruments is produced by an aluminum disc moving in a magnetic field.