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.