[Illustration: Fig. 503.--Effect of neighboring current upon a magnetic needle. Above the needle and parallel to it is a conductor carrying an electric current, the current flowing in the direction indicated by the arrow. This causes the north pole of the needle to turn toward the east. If the conductor be held _below_ the needle, its north pole will turn in the opposite direction or toward the west. These movements are easily determined by Ampere's rule as follows: _If a man could swim in the conductor with the current, and turn to face the needle, then the north pole of the needle will be deflected toward his left hand_.] Ques. Describe a simple galvanometer. Ans. It consists essentially of a magnetic needle suspended within a coil of wire, and free to swing over the face of a graduated dial. Ques. What is a galvanoscope and how does it differ from a galvanometer? Ans. A galvanoscope, as shown in fig. 504, serves merely to indicate the presence of an electric current without measuring its strength. It is an indicator of currents where the movement of the needle shows the direction of the current, and indicates whether it is a strong or a weak one. When the value of the readings has been determined by experiment or calculation any galvanoscope becomes a galvanometer. [Illustration: 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.