[Illustration: FIG. 129.--Electromagnetic induction: If a coil be given a simple motion of translation in a non-uniform or variable magnetic field, a current will be induced in the coil, whether the motion be from the dense to the less dense region of the field or the reverse, _because the number of lines of force passing through the coil is altered_.] 1. _To induce a current in a circuit, there must be a relative motion between the circuit and a magnetic field, of such a kind as to alter the number of magnetic lines embraced in the circuit._ 2. _The electromotive force induced in a circuit is proportional to the rate of increase or decrease in the number of magnetic lines embraced by the circuit._ For instance, if _n_ equal the number of magnetic lines embraced by the circuit at the beginning of the movement, and _n′_ the number embraced after a very short interval of time t, then the average induced electromotive force = (n - n′)/t It would require the cutting of 100,000,000 lines per second to produce an electromotive force equal to that of one Daniell cell. The unit of electromotive force, called the _volt_, is the electric pressure produced by cutting 100,000,000 lines per second, usually expressed 10^{8}. [Illustration: FIG. 130.--Experiment illustrating Lenz’s law which states that in all cases of electromagnetic induction, _the direction of the induced current is such as to tend to stop the motion producing it_. In the experiment, in order to produce the induced current, energy must be expended in bringing the magnet to the coil and in taking it away, which is in accordance with the law of conservation of energy.] 3. _By joining in series a number of conductors or coils moving in a magnetic field, the electromotive forces in the separate parts are added together._ The reason for this is apparent by considering a coil of wire having several turns and moving in a magnetic field so as to cut magnetic lines. During the movement, the lines cut by the first turn are successively cut by all the other turns of the coil, hence, the total number of lines cut is equal to the number cut by a single turn multiplied by the number of turns. The electromotive forces therefore of the separate turns are added. EXAMPLE--If a coil of wire of 50 turns cut 100,000 lines in 1/100 of a second, what will be the induced voltage? The number of lines cut per second per turn of the coil is 100,000 × 100 = 10,000,000. The total number of lines cut by the coil of 50 turns is 10,000,000 × 50 = 500,000,000. which will induce a pressure of 500,000,000 ÷ 10^{8} = 5 volts.