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Fig. 504.--Effect upon a magnetic needle of a neighboring current in a loop. In this arrangement the same conductor is simply carried back beneath the needle and hence both the upper and lower portions tend to turn it in the same direction, while the side branch or vertical section is ineffective. In accordance with Ampere's swimming rule, the upper wire causes the N pole of the needle to turn to the left, while if a man can imagine himself swimming in the lower wire in the direction of the current, and facing the needle (that is, swimming on his back), the N pole of the needle will turn to his left--that is to the east. The effect of the loop then has double the effect of the single wire in fig. 503.

Ques. For what use are galvanometers employed?

Ans. They are used for detecting the presence of an electric current, and for determining its direction and strength.

Ques. How is the direction and strength of the current indicated?

Ans. When a galvanometer is connected in a circuit, the direction of the current is indicated by the side towards which the north pole of the needle moves, and the current strength by the extent of the needle's deflection.

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.

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.

Fig. 507.--Breguet upright galvanometer with glass shade.

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.

NOTE.--Strong currents must not be passed through very sensitive galvanometers, for even if they be not ruined, the deflections of the needle will be too large to give accurate measurements. In such cases the galvanometer is used with a shunt, or coil of wire arranged so that the greater part of the current will flow through it, and only a small portion through the galvanometer.

Ques. What two kinds of coil are used?

Ans. The short coil and the long coil.

Ques. What is the difference between a short coil and a long coil galvanometer?

Ans. A short coil galvanometer has a coil consisting of a few turns of heavy wire; a long coil galvanometer is wound with a large number of turns of fine wire.

Fig. 509.--Bunnell galvanometer for measurements of instruments, lines, batteries, wires and any object from 1/100 to 10,000 ohms or more.

Ques. What is the action of short and long coil galvanometers?

Ans. With a given current, the total magnetizing force which deflects the needle is the same, but with a short coil, it is produced by a large current circulating around a few turns, instead of a small current circulating around thousands of turns as in the long coil. The short coil being of low resistance is used to measure the current, and the long coil with high resistance, is suitable for measuring the pressure. Hence, a short coil instrument with its scale directly graduated in amperes is an ammeter, and the long coil type with graduation in volts is a voltmeter.

Classes of Galvanometer.--There are numerous kinds of galvanometer designed