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is employed but one induction coil is needed for all cylinders, because the secondary or high-tension current from one unit is distributed to the spark plugs at the proper time. Various wiring diagrams will be presented to show the methods of using timers and distributors. It will be noticed that the high-tension portion of the distributor is well insulated from the primary circuit closing member at the lower end. This is necessary because current of high voltage is much more difficult to handle than that of lower pressure, and it is more liable to short circuit.

The arrangement of the contact points for various numbers of cylinders in roller contact timers is shown at Fig. 32. At A but one segment is provided, this obviously serving only one cylinder. The form depicted at B is utilized with a double-cylinder opposed motor or a twin-cylinder vertical type in which both connecting rods act on a common crank pin or crank pins in the same plane. As the explosions are evenly spaced and the intervals separating the sparks are equal, the contact segments are placed diametrically opposite and are separated by a space of 180 degrees. If the twocylinder engine is a vertical form having opposed cranks, the explosions will not be separated by equal intervals, so the segments must be placed to compensate for the difference which exists in the time interval separating the power impulses. Two contact segments are imbedded in the insulating ring, the contacts being separated by a space of 90 degrees on one side and 270 degrees on the other. This form of timer is seldom used at the present time because the two-cylinder engine of the pattern for which it is adapted has been practically discarded.

When three cylinders are used the contact points are separated by a space of 120 degrees as shown at D. In a four-cylinder timer the contact segments are spaced on quarters of the circle and are separated by a space equal to 90 degrees. With a six-cylinder motor six segments are necessary, these being separated by a space of 60 degrees, as shown at F.

Another form of timer is shown at Fig. 33. In this the contact is established between balls and a contact roller. In order to eliminate the wear that is unavoidable with plain bearing timers the casing carries ball bearings which are used to support the central hollow revolving member. Some timers of the form shown at Fig. 31, A, are fitted with a plain bearing which wears after the timer has been used and which produces irregular ignition due to a poor ground contact. Battery timers of the forms outlined are seldom used at the present time, as they have been succeeded by the more efficient short contact types. A notable excep

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Fig. 32.—Showing Disposition of Contact Points on Timers for Differing

Numbers of Cylinders. A-One Cylinder Type. B-Arrangement of Two Cylinder Opposed Motor. C—Contacts Separated by 90 Degrees in One Direction and 270 Degrees in the Other when Used on a Two Cylinder Vertical Engine with Opposed Crank Pins. D Three Cylinder Form. E-Spacing for Four Cylinder Engines. FType Employed on Six Cylinder Power Plant.

tion to this almost general rule is the Ford car, which is manufactured in immense quantities and which utilizes the roller contact timer previously described.

One of the best known of the short contact forms of timer is the Atwater-Kent, which is usually combined with a secondary distributor as shown at Fig. 35. The method of placing this timing and distributing member in circuit is clearly shown in

wiring diagram Fig. 34. The advantage of a timer of the form shown, as contrasted to the simple type previously considered, is that a one unit induction coil will serve any number of cylinders from 2 to 8, whereas with the roller type shown at Fig. 31 a separate induction coil is needed for each cylinder to be fired.

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Fig. 33.—Sectional View Showing Construction of Ball Bearing, Ball

Contact Timer.

It will be observed that the coil used with the Atwater-Kent system has five terminals, four of these being primary terminals, one at the center of the coil box a secondary or high tension terminal. A set of six dry cells connected in series is wired to one side of the coil box as indicated. One of the two remaining primary terminals runs to the primary contact at the bottom of the interrupter, the other to a grounding screw attached to the interrupter casing. The secondary terminal is connected to the central terminal of the distributor, while the remaining four terminals are joined to the plugs in the engine cylinders in such order as to insure proper sequence of explosions. The external view of the Atwater-Kent uni-sparker is shown at Fig. 35, A. In this a centrifugal mechanism is contained in the lower part of the

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Fig. 34–Wiring Diagram of Atwater-Kent Uni-Sparker.

casing by which the spark is automatically advanced as the speed of the engine increases.

The only points that will wear on a device of this character are the contact points which are clearly shown in the view of the contact breaker mechanism at Fig. 36. The revolving shaft in the center has a number of notches, two, three, four, six, or eight, according to the number of cylinders to be fired, cut into it. A light, hardened steel trigger, B, is held against the shaft at this point by a small spring. On turning the shaft this trigger is carried forward by the notches in the shaft, and is suddenly released as the hook end leaves the notch. In so doing the back of the trigger

strikes a small pivoted hammer, D, situated between the trigger and the spring carrying the contact points. This causes the contact points, K, to open and close with remarkable rapidity, but one contact being made for each spark. When it is desired to adjust the platinum contact points, as when they show signs of wear, it is only necessary to remove one or more of a number of extremely thin washers under the head of the adjustment screw and to replace

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Fig. 35.—Showing Construction of Atwater-Kent Uni-Sparker.

the screw.

The contact points should be absolutely clean and bright and have smooth contacting surfaces. The distributor portion of the device consists of a hard rubber block fitted to the top of the primary shaft, this carrying a brass quadrant that passes the high tension current to the spark plugs by means of the terminal points imbedded in the hemispherical cover. There is no actual contact between the rotating quadrant and the distributor points, as the high tension current is capable of jumping the very

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