=Ques. What is the behavior of a shunt dynamo when used as a motor?=
Ans. Its direction of rotation remains unchanged.
Ans. Because if the connections be such that the current supplied will flow through the armature in the same direction as when the machine is used as a dynamo, the current through the field will be reversed, since the field windings are in parallel with the brushes.
[Illustration: FIG. 411.--The "left hand rule" for direction of motion in motors. _Place the left hand_, as shown, _so that the thumb points in the direction of the current, the 3rd, 4th and 5th fingers in the direction of the lines of force, then will the 2nd or forefinger, at right angles to the others, point in the direction in which the conductor is urged_.]
=Armature Reaction in Motors.=--In the operation of a motor the reaction between the armature and field magnets distorts the field in a similar manner as in the operation of a dynamo. A current supplied from an outside source magnetizes the armature of a motor and transforms it into an electromagnet, whose poles would lie nearly at right angles to the line joining the pole pieces, were it not for the fact that _negative_ lead must be given to the brushes.
[Illustration: FIG. 412.--Principle of the electric motor as illustrated by experiment showing effect of a magnetic field on a wire carrying an electric current. Let a vertical wire _ab_ be rigidly attached to a horizontal wire _gh_, and let the latter be supported by a ring or other metallic support as shown, so that _ab_ is free to oscillate about _gh_ as an axis. Let the lower end of _ab_ dip into a trough of mercury. When a magnet is held in the position shown and a current from a cell is sent through the wire as indicated, the wire will move in the direction shown by the arrow _f_, that is, at right angles to the direction of the lines of magnetic force. Let the direction of the current in the wire be reversed, then the direction of the force acting on the wire will be found to be reversed also. The conclusion is that _a wire carrying a current in a magnetic field tends to move in a direction at right angles both to the direction of the field and to the direction of the current_. The relation between the direction of the magnetic lines, the direction of the current, and the direction of the force, is often remembered by means of the following rule, known as the _motor rule_, and which differs from the dynamo rule only in that it is applied to the fingers of the _left hand_ instead of to those of the right. _Let the forefinger of the left hand point in the direction of the magnetic lines of force and the middle finger in the direction of the current sent through the wire, then will the thumb, at right angles to the other two fingers, point in the direction in which the wire is urged._]