Ans. If a conducting circuit--a wire ring or single coil, for example--be moved in a uniform magnetic field, as shown in fig. 127, so that only the same number of lines of force pass through it, no current will be generated, for since the coil is moved by a motion of translation to another part of the field, as many lines of force will be left behind as are gained in advancing from its first to its second position. [Illustration: FIG. 127.--Electromagnetic induction: In order to induce a current by electromagnetic induction, a conductor must be so moved through a magnetic field _that the number of lines of force passing through it (that is, embraced) are altered_. If a coil be given a simple motion of translation in a uniform magnetic field as indicated in the figure, no current will be induced _because the number of lines of force passing through it are not changed_, that is, during the movement as many lines are lost as are gained.] =Ques. Describe another movement by which no current will be induced.= Ans. If the coil be merely rotated on itself around a central axis, that is, like a fly wheel rotating around a shaft, the number of lines of force passing through the coil will not be altered, hence no current will be generated. =Ques. State the essential condition for current induction in a uniform field.= Ans. The coil in which a current is to be induced, must be tilted in its motion across the uniform field, or rotated around any axis in its plane as in fig. 128, _so as to alter the number of lines of force which pass through it_. [Illustration: FIG. 128.--Electromagnetic induction: If a coil be given a motion of rotation from any point within its own plane so that it passes through a uniform magnetic field, a current will be induced in the coil _because the number of lines of force passing through it is altered_.] =Ques. In what direction will the current flow in the coil, fig. 128?= Ans. The current induced in the coil will flow around it in a clockwise direction (as observed by looking along the magnetic field in the direction in which the magnetic lines run) if the effect of the movement be to diminish the number of lines of force that pass through the coil. The current will flow in the opposite direction, (counter-clockwise) if the movement be such as to increase the number of intercepted lines of force. =Ques. If the magnetic field be not uniform, as in fig. 129, what will be the result?= Ans. The effect of moving the coil by a simple motion of translation from a dense region of the field to one less dense, or vice versa, will be to induce a current because in either case, the number of lines of force passing through the coil is altered.[12] =Laws of Electromagnetic Induction.=--There are certain laws of electromagnetic induction which, on account of the importance of the subject, it is well to carefully consider. The facts presented in the preceding paragraphs are embodied in the following fundamental laws: