flow through the loop, a connection would have to be made between the rings shown in the drawing. The drawings do not show an external connection, but the arrows indicate the direction the current would flow if an external connection was made. When the loop is in such a position as to be at right angles to the lines of force, as shown in (b), there will be no voltage generated in the loop, because the sides of the loop do not cut any lines of force when in this position. The same thing is true when the loop is rotated through a half turn, and side "B" is at the top and side "A" at the bottom. Therefore there is no voltage generated in the loop for two positions during each revolution. When the loop is in the position shown in (c), one half turn from position (a), the voltage generated in side "B" is such as to cause the current to flow into that side of the loop, and the voltage generated in side "A" is such as to cause a current to flow out of that side of the loop. Thus the direction of flow of the current through the loop reverses as it changes its position from (a) to (c).

When the sides of the loop are in the positions shown in (a) and (c), the number of lines of force cut for a given amount of rotation is greater than when the loop is in any other position. As the loop rotates from position (a) to position (b) the number of lines cut for equal amounts of rotation gradually becomes less until, at position (b), no voltage is generated, because when the loop is in position (b), the sides of the loop are moving along the lines of force instead of cutting them. From this it is seen that the voltage is continuously varying in magnitude and twice during each revolution the voltage is zero; also, the voltage changes direction twice for each revolution. The current generated by such a machine is called an alternating current because it alternately flows in one direction and then in the other. Two rings, one connected to each side of the loop, are provided, together with wiping contacts called "brushes" to permit external connections to the loop while it is in rotation. The rings are called "collector rings," or "slip rings." These rings are usually made of copper or brass, but iron is sometimes used. They are insulated from the shaft and from each other. The brushes are usually made of carbon or a combination of carbon and some metal.

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