=Secondary Induction Coils.=--The arrangement shown in fig. 137, may be considered as a very simple or rudimentary form of secondary induction coil. In the actual coil, the primary and secondary circuits (corresponding to 1 and 2 in fig. 135) are made up of coils of insulated wire, as shown in fig. 143, the primary coil P, being wound over a core C and the secondary coil S being wound over the primary. The one property of such an arrangement that makes it of great value for most purposes is that _the voltage of the induced currents may be increased or diminished to any extent depending on the relation between the number of turns in the primary and secondary winding._ This relation may be expressed in the following rule: _The voltage of the secondary current is (approximately) to the voltage of the primary current as the number of turns of the secondary winding is to the number of turns of the primary winding._ [Illustration: FIG. 137.--Production of spark with plain coil. Connect the ends or leads of the secondary winding to fixed insulators and bend the ends so they are from one-sixteenth to one-eighth inch apart. Connect one end of the primary winding to an electric battery, and with the other lead of the primary winding brush against the other terminal of the battery, as indicated. When the contact is broken there will be a spark both at the point of rupture in the primary circuit and at the gap. An electric impulse is also induced in the secondary circuit when the primary circuit is closed and the current flowing in it gradually rises to its maximum value, but this impulse is too feeble to cause a spark to jump across the gap. Only the impulse induced in the secondary during the dying out of the current in the primary is utilized.] For instance, if the voltage of the primary current be 5 volts, the primary winding have 10 turns and the secondary 100 turns, then Secondary voltage: 5 :: 100 : 10 from which Secondary voltage = 50 volts (approximately) The watts in each circuit are approximately the same; hence: if, for instance, the current strength in the primary circuit be 5 amperes, the watts in primary circuit are 5 × 5 = 25. Accordingly, for the secondary circuit the current strength is: 25 watts / 50 volts = 1/2 ampere (approximately) From this, it is seen that where the voltage is raised in the secondary circuit, the current flow is small as compared to that in the primary circuit; therefore, heavy wire is used in the primary winding and fine wire in the secondary, as indicated in figs. 137 and 143. For most purposes a very much higher secondary voltage is required than in the example just given.