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the revolving member of the timer turns at engine speed, and should be driven directly from and at the same speed as the crank shaft.

Simple timer forms suitable for one-cylinder motors are shown at Fig. 30. The simplest one, depicted at A, consists of a rocking member of fiber or other insulating material which carries a steel spring that is normally out of engagement with the surface of the cam. When the point of the cam brushes by the contact spring,

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Fig. 30.-Simple Forms of Contact Breakers or Timers Used on One Cylinder Engines. A-Wipe Contact. B-Touch Contact.

any circuit in which the device is incorporated will be closed and current will flow from the battery or dynamo to the transformer coils and spark plugs which are depended on to furnish a spark of sufficient intensity to insure ignition of the gas. It is desirable that the member which carries the contact spring be capable of a certain degree of movement, in order that the spark time may be advanced or retarded to suit various running conditions. In the form shown if the top of the casing is pushed in the direction. of the arrow, the contact spring will come in contact with the point of the cam which is turning in the direction indicated sooner than

it will if the base member is rocked in a reverse direction and the contact spring pulled away from the point of the cam instead of being moved forward to meet it. The wipe contact form is the simplest, but the spring is liable to wear at the point of contact and may break off and cause trouble. Such a device is more, suitable for low-speed engines that it is for those which have high crank-shaft velocity.

The single-cylinder timer depicted at B is a form that is widely used on high-speed engines and contact is made between a pair of platinum contact points which just touch each other instead of wiping. Platinum is a material that is not affected by the arcing or heat of the spark as much as steel or brass would be and provides a more positive contact. In the wipe contact form the continual brushing action of the cam against the spring tends to keep the contact surfaces clean, but this condition does not obtain in the simple touch contact of the form shown at B. The casing is rocked in the direction of the arrow to aavance a spark in either case. The form shown at B is more economical of current because the contact is shorter and is more suitable for high-speed engines. While the forms considered prove practical in their application to simple one- and two-cylinder engine forms, they are very heavy or clumsy appliances when used for four-cylinder engines, as it is very hard to assemble the spring elements so that the contact will take place at the proper point in all cylinders.

When a timer is to be used in connection with a four- or sixcylinder engine the compact form shown at Fig. 31, A, is usually adopted. This has many desirable features and permits of timing the spark with great accuracy. The contact segments are spaced on quarters for a four cylinder and are imbedded in a ring of fiber which is retained in a casing of aluminum. The central revolving element carries a lever which has a roll at one end and a tension spring designed to keep the roller in contact with the inner periphery of the fiber ring at the other. The segments are of steel and are accurately machined and hardened, and as the surface of the roller is also hardened, this form of timer is widely used because it provides a positive contact and works smoothly at all engine speeds.

A secondary distributor which is employed to distribute both high and low tension current is shown at Fig. 31, B. This consists of a primary timing arrangement in the lower portion, and a secondary current-distributing segment at the upper portion. The central revolving member carries as many rolls as there are cylinders to be fired, these being spaced at the proper points in the

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Fig. 31.-Timers Employed on Four Cylinder Engines. A-Four Contact Device for Commutating Primary Current. B-Combined Timer and Distributor, Directs Both High and Low Tension Energy.

circle to insure correct timing. One primary contact member is screwed into the casing, this contacting with the rolls as they revolve. At the upper portion of the case a number of terminals are inserted from which wires lead to plugs in the cylinders. When a timer of the form shown at A is used, a separate induction coil is needed for each cylinder and the number of units in the coil box and contact points on the timer will be the same as the number of cylinders to be fired. When a secondary distributor

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 thẻ 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. DThree 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

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