By hinging the rod pillars, or using a ball and socket joint, the discharger can be inclined so as to be better brought near the substance on the table.
The next important part of the coil is the contact breaker.
The armature R is a piece of soft iron carried at the end of a stiff spring, in about the middle of which, at B, is riveted a small platinum disk or stud. The adjusting screw A has its point also furnished with a piece of platinum, which is intended to touch the platinum on the spring when the latter is in its normal position. The core C of the coil serves as an electro-magnet. When the current flows from the battery (represented by the figure at L) through the primary coil and armature spring to the adjusting screw, it causes the armature to be drawn to the magnetized core, but thereby draws the platinum disk away from the adjusting screw. In so doing it breaks the circuit, the magnet loses its power, and the elasticity of the spring reasserting itself, carries the armature back, thereby reclosing the circuit. This is repeated many times in a second, the result being a continual vibration of the spring, and a consequent interruption to the current.
The condenser or Leyden jar J, connected as in the diagram to the base of the vibrating spring at K and to the adjusting screw wire M, is constructed as follows: On a sheet of insulated paper is laid a smaller sheet of tinfoil, one edge of which projects an inch or so over one end of the paper. Another sheet of paper covering this carries a second sheet of tinfoil, one end of which projects as in the first sheet, but at the opposite end of the paper. Tinfoil and paper sheets are laid in this manner alternately until a sufficient number is attained. The projecting ends are then clamped together and the whole pile immersed in melted paraffin, as will be described in a subsequent chapter. Wires are affixed to these clamped ends which serve to connect the condenser with the contact breaker. The conventional sign for a condenser is that used at J, showing the two series of plates, the insulation or dielectric, as it is called, being understood.
The size of condenser to use with different-sized coils varies according to the winding of the primary and the battery used. A primary coil of few turns would not necessitate as large a condenser as one of a large number of turns. At the same time, a condenser may be made of too great a capacity, and thereby weaken the action of the coil.
The base upon which the coil and its parts are mounted may be of dried polished wood. But where the coil is designed to give large sparks—over 2 inches—it is an advantage to use hard rubber one quarter of an inch and upward in thickness. Glass, were it not for the difficulty of drilling it and its brittleness, would be a desirable material for a coil base in a dry atmosphere. Hard red or black fibre coated with shellac varnish is also serviceable, and, moreover, is extremely easy to work. Slate must never be used; there is too much liability of iron veins being found in it, which in such high tension experiments as will be described would seriously impair the usefulness of the apparatus. The material selected for the base must be one that will not absorb moisture. A paraffined surface collects moisture up to a certain point in isolated drops, whereas a glass and even a hard rubber surface condenses the moisture as a film, which latter is extremely undesirable. But unfortunately the fact that a paraffined surface does not present a pleasing appearance would probably result in its rejection. And lastly, by mounting the coil on hard rubber blocks, or extending the reel ends to raise the coil body, a high insulation can be obtained at the sacrifice perhaps of appearance or height. From the care taken to insulate the secondary coil, it may be considered a superfluous precaution to so carefully select a base, but practical work with the instrument at some important crisis will demonstrate the necessity of extreme care in the smallest details relating to insulation. It may be well to note here that hard rubber is acted upon by ozone, and is thereby impaired as an insulator.
Fig. 10.
The base forms the top of a flat box in which the condenser lies; but there are a few points worth considering right here. As the connections of the coil will probably be under the base, a sufficient space must intervene between the base and the top of the condenser. It is a good plan to lay the condenser at least one half inch below the top of this box, and fill up to, say, one eighth of an inch with melted paraffin, leaving the condenser wires projecting for attachment. The connections of the primary coil and contact breaker should by all means be soldered, not simply wires held under screw nuts. And, moreover, all wires under the base should be so run that they do not cross one another, which precaution only requires a little planning. Then, when the connections are all made and the base laid on top of the box, it can be pressed down if the paraffin be warm, so that the screw heads and wires mark out their own channels and cavities in which to lie.