[Illustration: Fig. 505.--Effect upon a magnetic needle of a neighboring current in a coil. The coil as shown, is equivalent to several loops, that is, the force tending to deflect the needle is equal to that of a single loop multiplied by the number of turns. Hence, by using a coil with a large number of turns, a galvanometer may be made very sensitive so that the needle will be perceptibly deflected by very feeble currents. An instrument, as shown in the figure is called a _galvanoscope_. When it is accurately constructed, and supplied with a scale showing how many degrees the needle is deflected it is then called a galvanometer.] Ques. How should a galvanometer be set up before using? Ans. When no current is flowing, the coil should be parallel to the magnetic needle when at rest. Ques. What is a "sensitive" galvanometer? Ans. One which requires a very small current or pressure to produce a stated deflection. It does not follow that a galvanometer which is sensitive for current measurement will also be sensitive for pressure measurement. [Illustration: Fig. 506.--Bunnell simple detector galvanometer. It has middle clamps and scale divided into degrees.] Ques. Define the term "sensibility." Ans. With reference to mirror reflecting galvanometers it may be defined in three ways. First, in _megohms_, the sensibility being the number of _megohms_ through which one volt will produce a deflection of one millimeter with the scale at one meter distance. Second, in _micro-volts_, the sensibility being the number of micro-volts which applied directly to the terminals of the galvanometer will produce a deflection of one millimeter on a scale one meter from mirror. The sensibility is best stated in megohms for high resistance galvanometers and in micro-volts for low resistance galvanometers, and is frequently given both for galvanometers for intermediate resistance. Third, in micro-amperes, the sensibility being the number of micro-amperes that will give one millimeter deflection with scale at a distance of one meter. Ques. Upon what does the sensibility depend? Ans. 1, Upon the number of times the current circulates around the coil, 2, the distance of the needle from the coil, 3, the weight of the needle, 4, the current strength, and 5, the amount of friction produced by its movement. [Illustration: Fig. 507.--Breguet upright galvanometer with glass shade.] [Illustration: Fig. 508.--Bunnell horizontal galvanometer. It has two coils, one of which is of zero resistance and one of fifty ohms resistance adapting it to a variety of test.] The needle is usually quite small, and often a compound one. In very sensitive galvanometers, the coils are wound with thousands of turns of very fine wire, and shunts are generally used in connection with them.