Moving Coil Galvanometers,— This type is most familiar in the d'Arsonnal galvanometers, and is to be preferred for most purposes within the limits of its sensitiveness. Its preference lies in the fact that it may easily be made dead beat, and the readings are practically independ ent of the earth's field or magnetic substances in its vicinity. The construction also adapts itself to a portable type of instrument, and it may be so constructed as to be very slightly affected by vibrations. The sensitiveness of this class of galvanometers may be further in creased by substituting an electromagnetic field for that produced by permanent magnets.
Among the newer European galvanometers are the following: The Broca galvanometer is of the type having fixed coils and a moving magnetic system. Its distinctive feature is the form of this system, which consists of two steel wires hung vertically and close together. Each wire has like poles at its ends, making the system astatic. The Einthoven string gal vanometer has a single fine wire or quartz fibre in the intense field of a powerful electro magnet having a narrow air gap. When a cur
rent flows through the string, the latter moves across the field. The motion is observed with a microscope or is registered by projecting the image of the string on a screen. This galvan ometer has an extremely short period, as low as one one-hundredths of a second. It is highly sensitive and is free from inductance and capac ity. Duddell thermo-galvanometer has sus pended in the field of a permanent magnet a loop of silver wire which is closed at the bottom by a thermocouple of bismuth and antimony. The current to be measured is caused to flow through a heating coil placed below the thermocouple. The heat sets up a current in the loop and deflects it more or less according to its strength. This galvanometer works equally well with either direct or alternating currents. It can be used with the wireless cur rents and for the telephone. Another type called the Duddell oscillograph is made for voltages up to 50,000 in either of two forms— with permanent magnet and with electromag net. The former is preferred for the high frequency currents and is more readily trans ported when necessary.