The voltage depends on three things:--(1.) The _strength_ of the magnet: the stronger it is, the greater the number of lines of force coming from it. (2.) The _length_ of the conductor cutting the lines of force: the longer it is, the more lines it will cut. (3.) The _speed_ at which the conductor moves: the faster it travels, the more lines it will cut in a given time. It follows that a powerful dynamo, or mechanical producer of current, must have strong magnets and a long conductor; and the latter must be moved at a high speed across the lines of force. A SIMPLE DYNAMO. In Fig. 67 we have the simplest possible form of dynamo--a single turn of wire, _w x y z_, mounted on a spindle, and having one end attached to an insulated ring C, the other to an insulated ring C^1. Two small brushes, B B^1, of wire gauze or carbon, rubbing continuously against these collecting rings, connect them with a wire which completes the circuit. The armature, as the revolving coil is called, is mounted between the poles of a magnet, where the lines of force are thickest. These lines are _supposed_ to stream from the N. to the S. pole. In Fig. 67 the armature has reached a position in which _y z_ and _w x_ are cutting no, or very few, lines of force, as they move practically parallel to the lines. This is called the _zero_ position. [Illustration: FIG. 67.] [Illustration: FIG. 68.] In Fig. 68 the armature, moving at right angles to the lines of force, cuts a maximum number in a given time, and the current induced in the coil is therefore now most intense. Here we must stop a moment to consider how to decide in which direction the current flows. The armature is revolving in a clockwise direction, and _y z_, therefore, is moving downwards. Now, suppose that you rest your _left_ hand on the N. pole of the magnet so that the arm lies in a line with the magnet. Point your forefinger towards the S. pole. It will indicate the _direction of the lines of force_. Bend your other three fingers downwards over the edge of the N. pole. They will indicate the _direction in which the conductor is moving_ across the magnetic field. Stick out the thumb at right angles to the forefinger. It points in the direction in which the _induced_ current is moving through the nearer half of the coil. Therefore lines of force, conductor, and induced current travel in planes which, like the top and two adjacent sides of a box, are at right angles to one another.