After the _primary winding_ has been wound over the insulated core, and the ends have been properly brought out through the heads of the spool to be connected to binding posts thereon, a layer of insulating material is applied over the primary wire, and the secondary winding is then wound on.

The wire for the secondary winding consists of about No. 36 B and S gauge silk covered magnet wire, the amount used varying considerably, depending on the desired voltage of the secondary current.

When all the wire has been wound on, the ends are brought out to the binding posts, the coil is soaked in sh.e.l.lac dissolved in alcohol and baked, or in melted paraffin or a paraffin compound, and allowed to cool. It is then placed in either a cylindrical hard rubber sh.e.l.l or in a hard wood box.

The proportions of such coils vary greatly; for motor cycle use they are made long and of small diameter (102-1/2 inches for instance), while for some other purposes short and thick coils are found more convenient.

=Ques. How may the coil just described be connected for demonstrating purposes?=

Ans. Connect the ends of the secondary winding to fixed insulators and bend the ends so they are about 1/8 inch or less apart. Connect one end of the primary winding to a battery and brush the other end of the primary winding against the other terminal of the battery as indicated in fig.

137.

=Ques. What happens when the primary circuit is made?=

Ans. An electric pressure is induced in the secondary circuit, but of not enough intensity to cause a spark to jump across the air gap.

[Ill.u.s.tration: FIG. 139.--A Medical coil with armature and attachments consisting of electrodes, foot place, sponge, induction coil etc. A current of any degree of intensity may be obtained. The currents furnished are: 1, primary, 2, secondary; and 3, primary and secondary combined.]

=Ques. What happens when the primary circuit is suddenly broken?=

Ans. A spark is produced both at the point of break in the primary circuit and at the air gap in the secondary circuit.

=Ques. Why is a spark produced at the air gap at break and not at make of the primary current?=

Ans. Because when the current is flowing it cannot be stopped instantly on account of self-induction, that is, it acts as though it possessed weight.

If the reader has charge of a gas engine with a make and break ignition system, he will often avoid vexatious delays in locating ignition troubles, if he remember that one of the most important conditions for obtaining a good spark is that the _break take place with great rapidity_. This, of course, involves that the ignition spring be adjusted to the proper tension.

[Ill.u.s.tration: FIG. 140.--Rhumkorff induction coil. A secondary coil with vibrator and condenser; a type generally used in the laboratory. The name Rhumkorff was formerly very widely applied to induction coils for the reason that some of the earliest coils were constructed by Rhumkorff.]

=Secondary Induction Coils with Vibrator and Condenser.=--A plain secondary coil, such as just described, will only give feeble sparks for its size for the following reasons: The inductive effect of the primary winding in the secondary depends as previously explained on the rate at which the current in the primary winding decreases or dies out.

If a strong inductive effect is to be produced in the secondary, the current in the primary must stop suddenly. This is prevented by self-induction in the primary winding, which opposes any change in the current strength. The direct result is that, as the primary circuit is broken, a spark appears at the break, which means that the current continues to flow after the break has occurred, dying down comparatively slowly, hence, the inductive effect on the secondary winding is small.

[Ill.u.s.tration: FIG. 141.--Conventional diagram of a condenser. A condenser is a device designed to absorb or hold an electric charge in about the same manner as a vessel will hold a liquid. Every conductor of electricity forms a condenser and its capacity for holding a charge depends upon the extent of its surface. A condenser is therefore made of conductive material formed into such shape as to present the maximum surface for a given amount of material.]

The spark at the break in the primary circuit is even larger than that in the secondary circuit, and as this primary spark serves no useful purpose, but, on the contrary, quickly burns away the contact points, such an arrangement is obviously defective.

The vibrator-condenser coil is designed to overcome this trouble and also to give a series of sparks following in rapid succession instead of one.

It should be noted that a series of sparks following each other with considerable rapidity may be obtained with a plain coil by placing a _mechanical vibrator_ in the primary circuit, as used on some motor cycle ignition circuits.

The object of the vibrator, of a vibrator-condenser coil, is to rapidly make and break the primary circuit during the time the primary circuit is closed externally. It consists of a flat steel spring secured at one end, with the other free to vibrate. At a point about midway between its ends, contact is made with the point of an adjusting screw, from which it springs away and returns in vibrating. The points of contact of blade and screw are tipped with platinum. One wire of the primary circuit is connected to the blade and the other to the screw, hence, the circuit is made when the blade is in contact with the screw and broken when it springs away.

[Ill.u.s.tration: FIG. 142.--Construction of condenser for an induction coil.

The conducting material used is tinfoil, of which a large number of sheets are prepared, all cut to the same size. These are placed, one on top of the other, like the pages of a book, with a thin layer of insulating material between, usually two sheets of paraffined paper. Numbering the successive sheets of tinfoil serially, all sheets of even number are connected together and all sheets of odd number are connected together, these connections forming the terminals of the condenser. The condenser is then connected across the break in the primary circuit.]

A condenser is used to absorb the self-induced current of the primary winding and thus prevent it opposing the rapid fall of the primary current.

Every conductor of electricity forms a condenser and its capacity for absorbing a charge depends upon the extent of its surface. Hence, a condenser is constructed of conductive material so arranged as to present the greatest surface for a given amount of material.

The usual form of condenser for induction coils as shown in figs. 141 and 142 is composed of a number of layers of tin foil separated by paraffin paper, each alternate layer being connected at the ends.

Fig. 143 is a diagram of a vibrator coil. CC represents the core composed of soft iron wires. PP is the primary winding and SS the secondary. There is no connection between these windings and they are carefully insulated.

Y is the vibrator or _trembler_ and D the center about which it vibrates.

W is a switch used for opening and closing the primary circuit; B, a battery of five cells. The point of adjusting screw A rests against a platinum point R soldered upon the vibrator.

[Ill.u.s.tration: FIG. 143.--Diagram of a vibrator coil. The parts are as follows: A, contact screw; B battery; C, core; D, vibrator terminal; G, condenser; P, primary winding; S, secondary winding; W, switch; Y, vibrator. When the switch is closed, the following cycle of actions take place: 1, the primary current flows and magnetizes core; 2, magnetized core attracts the vibrator and breaks primary circuit; 3, the magnetism vanishes, including a momentary high tension current in the secondary winding; 4, magnetic attraction of the core having ceased, vibrator spring renews contact; 5, primary circuit is again completed and the cycle begins anew.]

If the switch W be closed, the electric current generated by the battery B will flow through the primary winding. This will cause the core CC to become magnetized, and the vibrator Y will at once be drawn toward it.

This will break the connection at R. The core, being made of soft iron, immediately upon the interruption of the current, will again lose its magnetism, and the vibrator will return to its original position. This again closes the circuit, after which the operation of opening and closing it is repeated with great rapidity so long as the switch W remains closed.

[Ill.u.s.tration: FIG. 144.--Circuit diagram of a master vibrator coil. B, is the battery; C, the unit coils; C1, C2, etc., the condensers; P, the primary windings and S, the secondary windings; H1, H2, etc., the spark plugs; T, the timer; MP, the master primary; V, the vibrator; W, the common primary connection; 1, 2, etc., the stationary contacts of the timer.]

The cycle of actions may be briefly stated as follows:

1. A primary current flows and magnetizes the core;

2. The magnetized core attracts the vibrator which breaks the primary circuit;

3. The core loses its magnetism and the vibrator springs back to its original position;

4. The vibrator, by returning to its original position, closes the primary circuit and the cycle begins again.

[Ill.u.s.tration: FIG. 145.--The Splitdorf master vibrator coil. As shown in the ill.u.s.tration the several unit coils are indicated by the figures 1, 2, 3, and 4. A fifth unit V at the left contains the master vibrator. The primary wires P connect with the timer and the secondary wires S with the plugs. B B shows the battery connections.]

=Magnetic Vibrators.=--Many types of vibrator are used on induction coils, the most important requirement being that _the break occur with great rapidity_. In order to render the break as sudden as possible, different expedients have been resorted to, all tending to make the mechanism more complicated, yet having sufficient merit in some cases to warrant their adoption.

In the plain vibrator, the circuit is broken at the instant the spring begins to move, hence, the operation must be comparatively slow.

In order to render the break more abrupt some vibrators have two moving parts, one of which is attracted by the magnetic core of the coil and moved a certain distance before the break is effected. A vibrator of this type is shown in fig. 146 and described under the ill.u.s.tration.

[Ill.u.s.tration: FIG. 146.--A hammer vibrator. When at rest, the upward tension of the spring, which carries the armature A, holds the platinum points in contact and causes the upper spring C, to leave shoulder of adjusting screw D, and rest against the heavy bra.s.s plate above it. When the iron core B, attracts the armature A, the downward tension on the upper spring, C, causes the latter to follow the armature down, holding the platinum point in contact, until the end of the upper spring C, strikes the lower shoulder of the adjusting screw, D, which gives it a "hammer break." The adjusting screw is held firmly in position by a bronze spiral spring under shoulder D.]

=Vibrator Adjustment.=--When a vibrator coil is used, the quality of the spark depends largely upon the proper adjustment of the vibrator. The following general instructions for adjusting a plain vibrator should be carefully noted:

1. Remove entirely the contact adjusting screw.

2. See that the surfaces of the contact points are flat, clean and bright.

3. Adjust the vibrator spring so that the hammer or piece of iron on the end of the vibrator spring stands normally about one-sixteenth of an inch from the end of the coil.

4. Adjust the contact screw until it just touches the platinum contact on the vibrator spring--be sure that it touches, but very lightly. Now start the engine; if it miss at all, tighten up, or screw in the contact screw a trifle further--just a trifle at a time, until the engine will run without missing explosions.

TABLE OF INDUCTION COIL DIMENSIONS.

Length of spark | 3/8 inch | 1/2 inch | 1 inch | 2 inches ------------------------------------------------------------------------ Size of bobbin | 2-1/8 | 2-1/2 | 3 3/8 | 4 2-3/4 ends | 1-1/4 | 5/16 | | 3/8 ------------------------------------------------------------------------ Length of bobbin | 4 | 5-1/2 | 6-1/2 | 6-1/2 ------------------------------------------------------------------------ Length and | 4-1/4 | 6 5/8 | 6-1/2 3/4 | ---- diameter of core | 7/16 | | | ------------------------------------------------------------------------ Size of base | 7-1/4 | 9 5 2 | 14-1/2 | 12 7-1/2 | 3-1/4 | | 6 | 3-1/4 | 1-1/2 | | 1-3/4 | ------------------------------------------------------------------------ Size of tinfoil | 4 2 | 5-1/2 | 6 4 | 6 6 sheets | | 3-1/4 | | ------------------------------------------------------------------------ Number of tinfoil| 36 | 40 | 40 | 60 sheets | | | | ------------------------------------------------------------------------ Size of paper | 5 3 | 6-1/2 | 9 5 | ---- sheets | | 4-1/4 | | ------------------------------------------------------------------------ Primary coil | No. 18 | No. 18 |2 layers No. | 2 layers 14 | | |16, silk | B. W. G. silk | | |covered. | covered.

------------------------------------------------------------------------ Secondary coil | 3/4 lb. | 1 lb. | 1-1/4 lbs. | 2-1/2 lbs.