[Illustration: Figs. 532 and 533.--Diagrams showing essential features of construction and principle of operation of D'Arsonval galvanometer.] Ques. Upon what does the sensitiveness of the instrument depend? Ans. Upon the strength of the field of the permanent magnet, the number of turns in the suspended coil, and the torsion of the wires by which it is suspended. Ques. When is this galvanometer called "dead beat"? Ans. When the construction is such that the moving part comes quickly to rest without a series of diminishing vibrations. [Illustration: Figs. 534 to 536.--Queen horizontal magnet D'Arsonval galvanometer with telescope and scale. It is very sensitive and is used in many electrical measurements, including commercial testing, such as measuring insulation of cables, fault location, etc. It is not affected by surrounding magnetic disturbances, and may, therefore, be used in proximity to dynamos and switchboards. The instrument has a pair of binding post terminals, one of which connects to a bottom spiral of the system and the other forms a junction with the top of the tube holding the system, forming a complete circuit through the coil. The tube containing the system may be readily removed from the magnet and another tube having a different system inserted as is required for various kinds of electrical measurement. The entire system with its suspension may be inspected by the removal of a thumb screw. To inspect interior of tube first be sure that the screw B is turned so that the coil is clamped. Entirely remove screw C, and, holding the outside tube near the window, press firmly with the finger on the extreme top of the suspension support. The inside rib, with complete suspension, will draw from the tube, and the working parts can be fully inspected. Carefully return same to its original position in tube, setting tight the screw C. The galvanometer is designed so that the coil is clamped in position when the galvanometer must be transported. The insulation of the galvanometer terminals and binding posts is such as to guard against any possible leakage. As a further protection, each levelling screw is provided with a hard rubber insulator. This feature is essential since, in making insulation measurements, the operator wishes to be assured that the deflection being obtained is the result of leakage upon the cable or wire being measured and not leakage between the galvanometer terminals. The galvanometer is provided with an attached telescope and scale for noting the deflections. The deflections produced by this galvanometer are proportional to the current. To facilitate quickly setting up the instrument, two way levels are provided.] Ques. What causes this? Ans. The instrument is made dead beat by winding the coil on a copper or aluminum frame, so that when in operation, currents are induced in the frame by the motion of the coil in the magnetic field; these currents oppose the motion of the coil.