John E. Atkins
It is sometimes disappointing to the amateur, after assembling primary and secondary of a coil designed for a three or four inch spark to find, in spite of all the primary current available, that the coil is not giving its rated spark. Oft-times the amateur is at loss to determine whether the difficulty is in his own work or in the materials purchased. In many cases, coil failings may be rightly attributed to defects in the home made interrupting device, as the making of vibrators and interrupters is found to be a study in itself.
The first thing to be considered is the frequency of vibration or, in other words, how long shall the spring of the vibrator be, and with a certain length what will be its speed ? Any amateur who has experimented with spring metal or even whalebone, understands that with a certain thickness and width of spring metal, the longer the piece in vibration the slower its speed and vice versa, the shorter the piece the faster it vibrates. Presuming that we have a piece of thin spring-brass in use as a vibrator, consider that the piece vibrates 700 times a minute. Then, with proper platinum contacts, the primary current is interrupted in its progress through the primary winding just 700 times each minute. Now 1-700 of a minute seems a very small period of time to measure, and it is at first difficult to perceive how any good can come from such a brief contact, and in the majority of vibrators using two platinum contact points, this is about the limit for maximum results from a winding, unless the secondary in use is wound for a much greater spark capacity and then underrated, which is, of course, a waste of fine wire. Small coils, such as 1/4 to 3/4-in. sizes, permit of a much faster vibrator, and on auto coils the speed is often 1000 and higher.
It must be borne in mind that the writer speaks of vibrating interrupters, and not of rotary and other forms involving other principles of interruption.
It will be found by experiment that there is a certaint speed of interruption best suited for each coil, and with the assistance of the following description and diagrams the amateur should be able to get one-fourth to one-half more spark capacity out of any home-made coil, than where a core-actuated movement is brought about by a separate or independent electro-magnet in series with the primary winding.
The first requisite is a base-board of 1/2-in. stock, white-wood or mahogany, size 4 by 7 in. The electromagnet is of 3/8 in. soft iron, 3 in. long, drilled and tapped at one end for an 8-32 machine screw by which the magnet is attached to the base board. The fiber washers are 1 in. in diameter. This magnet is wound with No. 20 double cotton covered wire. The vibrating spring is of No. 20 gauge steel or spring brass, 1/2 in. wide and 3 1/2 in. long. The ham er head is of iron, 3/4 in. wide and | in. thick, scldered or pinned to one end of the spring. The other end of the spring has a 1/8 in. hole for affixing to the standard by means of a long machine screw that passes through a hole in the standard through the base board, where it is held securely by a nut. The hammer head is now 1/4 in. clear and above the top of the electro-magnet. The standard above mentioned is cut from a piece of § brass rod.
The contact-screw supports consists of a stout strip of brass rod affixed to two upright rods in a manner similar to the spring support. One platinum point is set into or soldered to the contact-screw. The other platinum point or stud is not soldered to the spring it-self, but to a thin spring of brass riveted to the vibrating spring. (See sketch.) The purpose of this method is to supply means of giving an instantaneous break to the interruption The value of the ascendency spark depends considerably on this quickness of break. In operation, the hammer head is attracted toward the magnet and has travelled nearly a 64th of an inch, and is going very fast when the platinum spring is "kicked" by the stud borne by the vibrating spring, and released from the set-screw contact. This break is far superior to any other simple method and should be adopted for all home vibrators.
In making coil connections, one terminal of the magnet coil goes to the terminal of the primary winding. The other terminal of the primary is connected to the battery. The other side of the battery goes to the vibrator post. The second end of the magnet coil goes to the contact. (See diagram.)
The condenser to be used on this vibrator is connected, one terminal to the contact screw and one to the pillar post. The capacity of this condenser is ascertained by experiment. It is assembled, not by mathematical calculation as to micro-farads, but is built up until the condenser seems to produce the best secondary discharge. Every amateur knows that a condenser is necessary to get a spark, but not every one understands fully its functions. One thing is certain, by a careful adaptation of the condenser, the best secondary spark will be obtained when the platinum point spark is almost at a minimum. Too much condenser will reduce this contact point sparking to nearly nothing, to the detriment of the secondary results. It will therefore be seen that too much condenser is a harmful possibility; also a poorly constructed condenser is injurious and, furthermore, there never yet has been a coil vibrator of a reliable sort but there is some sparking at the points, and while this sparking in the laboratory is of little consequence, on a power boat or an auto it is a nuisance, and coil manufacturers look more to the efficiency of their vibrator, as regards minimum sparking and consequent non-welding of platinum contacts, than to any other detail of coil construction.
It is the writer's belief that the foregoing interrupter is applicable to almost any battery coil giving three and four inch sparks. The design furnishes excellent •opportunity for adjusting the length of the vibrations by merely drilling long slots in the spring and in the base board so that the pillar-post may be brought nearer the magnet.