by means of a very rapid automatic magnetic circuit breaker or vibrator, the action of which has been previously considered.

Q. Describe timer construction.

A. The usual form of timer as outlined at Fig. 81 is composed of a fiber insulating ring which carries a number of metal contacts which are out of electrical connection with each other and with the metal body of the timer. The rotary contact member revolves in the interior of the insulating fiber ring and makes contact with the segments as it revolves. Each segment communicates with one of

Fig. 85.-How Current is Induced in Secondary Coil by Varying Flow of Energy Through Primary Coil.

the units of an induction coil and the number of segments and terminals used is determined by the number of cylinders to be fired. The device shown is adapted for a four cylinder motor.

Q. What is the difference between a single cylinder engine timer and one for multiple cylinder ignition?

A. The difference is only in the number of insulated contacts. The timer intended for the multiple cylinder engine having the greater number. A one cylinder timer has but one insulated contact segment and one external primary terminal. As the number of

cylinders increase additional contacts are provided, one for each added cylinder.

Q. Are timers used only with primary current?

A. Timers may be used in connection with either primary or secondary current, though when used for distributing the high tension current, they are termed "secondary distributors.

Q. What is the difference between a primary timer and a secondary distributor?

A. In a primary timer the revolving contact element is grounded or attached directly to the metal parts of the engine. In a secondary distributor the revolving brush or contact member is well insulated from the engine and is coupled directly to the source of current or secondary winding of the induction coil. Special care must be taken to insulate the terminals and contacts of a high tension distributor on account of the ease with which the high voltage current overcomes resistances that would effectively bar the progress of the low tension current. Usually a secondary distributor includes a primary timing arrangement as well when used in connection with a battery ignition system.

Q. How are contacts in timers and distributors spaced?

A. In a timer intended for a two cylinder motor the contacts are spaced according to the arrangement of the cranks on the crankshaft. If the motor is a double opposed form or a two cylinder vertical type with both connecting rods attached to a common crank pin, the contacts are separated by 180 degrees or are spaced on halves of the circle. If the engine is a two cylinder vertical or V type, the contacts are not evenly divided but are separated by spaces of 270 and 90 degrees respectively. The contacts of a three cylinder timer are spaced on thirds of the circle or 120 degrees apart. Those of a four cylinder timer are separated by a space of 90 degrees and are spaced on quarters of the circle. (See Fig. 81.) For a six cylinder engine the contacts are spaced on sixths of the circle, which means that they are 60 degrees apart.

Q. What is the difference between primary and secondary wire and why are they different?

A. Primary wire carries current of low voltage that does not

short circuit easily, so the insulation is not nearly as heavy as that on the secondary or high tension current conductors. A primary wire is insulated with but one layer of rubber; whereas a secondary wire has several of insulating compound. A number of wires are shown at Fig. 86. Those at A, B, and C are heavily insulated conductors used to convey high tension energy, while the wires with the lighter insulation, as shown at D and E, are suitable for primary circuits. A number of primary leads are sometimes joined together and though separately insulated, are run through a common conduit, or insulating tube, as shown at F. When used in this manner the inner insulation of the different wires is usually of different colors so they may be easily recognized when making connections.

Q. How is electricity measured?

A. The strength, quantity, potential and other characteristics of the electric current may be readily determined by simple meters provided with a pointer and a calibrated or graduated scale so the readings can be made easily.

Q. What is the volt?

A. The volt is the unit of pressure and may be taken as corresponding to the head or pressure of water.

Q. What is the ampere?

A. The ampere is the unit of current quantity and corresponds to the amount of electricity flowing through a circuit. A current may have a large amperage, which means that a large volume is flowing, and yet it may have low pressure or voltage. The reverse condition may also obtain and a current of high voltage may not have much amperage.

Q. What is the watt?

A. The watt is the unit indicating the amount of electricity or the value of the current flow. A current of one ampere flowing at a potential or pressure of one volt has a value of one watt. There are 746 watts to the electrical horse-power. In all cases the amount of current consumed by any piece of electrical apparatus is indicated in watts, which is always the product of the number of amperes of current times the potential or voltage.

Q. What is the ohm?

A. The ohm is the unit by which the resistance anything offers to the passage of the electric current is measured. To overcome one ohm of resistance it will take a current of one ampere at a pressure of one volt.

Q. How are dry batteries measured?

A. Dry batteries are measured as shown at Fig. 87 by means of a simple device which measures the amperage or current output, and which is called an "ammeter." A standard 6"x21/2" dry cell in good condition should indicate a current strength of from 20 to 25 amperes. The voltage of a dry cell varies from 1.25 to 1.5 volts.

ductors.

Q. How is the current strength of storage batteries determined?

A. Storage batteries are measured with a voltmeter, which is an instrument similar in construction to an ammeter with the exception that it is designed to register the pressure instead of the quantity of the electric current flowing through it. A single cell of a storage battery is supposed to test 2.2 volts when fully charged, which means that a 2 cell battery will indicate 4.4 volts while a 3 cell battery shows 6.6 volts. The ordinary form of battery testing ammeter should never be used in testing a storage battery, he

cause its construction is such that it will practically short circuit the battery and it may produce buckling of the plates owing to the rapid discharge of the stored energy.

Q. How can current consumption of induction coil be found? A. The method of

AMME

15 20 25

using a simple current indicator to show the amount of electricity used by an induction coil is clearly outlined at Fig. 88. The instrument shown is a low reading ammeter, and is so arranged that the terminal end may be inserted between the switch button and switch lever, while the coil is in operation. The amount of current drawn by each unit can be easily determined by testing out but one unit at a time. This can be easily done by raising the contact screws of the vibrators so that they do not touch the contact on the vibrator blade, except on the unit that is to be tested. The engine is then cranked over by hand and the amount of current consumed by the coil vibrator will be clearly indicated as soon as contact is established at the timer. The ordinary induction coil unit consumes from .5 to 1 ampere, and if the current consumption is greater than this, the tension of the vibrator spring should be lessened so that less current will be necessary to energize the primary winding.

of Testing

Fig. 87.-Method
Current
Strength of Dry Cell With Ammeter.