Fig. 59. Spark Gaps.
The best spark gap for use with a small untuned transmitter, making use of neither tuning helix nor condenser, is shown at the left in Fig. 59. It consists of two brass balls supported on suitable double binding posts. The balls tend to thicken the spark and make it more disruptive than if it passed between rods or points. The balls also increase the potential required to leap the gap without increasing its length and permit the aerial to become more highly charged before a discharge takes place. For this reason the balls or knobs must always be kept free from small rough spots or points by frequent polishing.
Silver has some peculiar property which makes it the most efficient material for a spark gap, but its cost prohibits its use and so brass or zinc, which are next in order, is generally used.
The type of gap illustrated at the right in Fig. 59 is very convenient and efficient for small tuned transmitters employing coils of low power.
Fig. 60. Spark Gap.
The zinc tips are pieces of 3/8-inch zinc battery rod 3/4 inch long. They are bored and threaded to fit a pair of brass rods 1/8 inch diameter and 2 1/2 inches long. The rods are supported by two double binding posts and are fitted with two pieces of hard rubber rod 1/2 inch diameter and 1 inch long to serve as handles. If desirable one of the rods and binding posts may be threaded so that the adjustment of the gap can be accomplished by revolving the handle and screwing the gap in or out.
A gap of the size and type just described would not be suitable in connection with a large transformer or an induction coil. The heavy discharge of such powerful instruments would very rapidly heat the small brass parts and corrode or even melt the zinc. The spark gap such as is shown in Fig. 59 may be used with a very large coil or the two kilowatt transformer if the same proportions are kept and it is made one-half again as large. Using the dimensions shown in the figure, the spark gap is suited to either the open core or the 1/4 and 1/2 kilowatt transformers.
Two pieces of hard rubber rod, 3/4 inch diameter and 2 inches long serve as the standards. Three grooves 1/8 inch wide, 1/8 inch deep and 1/8 inch apart better the appearance and reduce the liability of leakage but are not necessary. Two holes 2 inches apart are bored in S, a strip of brass 3 x 3/4 x 1/4 inches. Two screws pass through these holes into the rubber standards BB and hold S firmly in position. The end of S is bent down at right angles and bored to receive a binding post. A hole is bored in S halfway between the standards and tapped to receive a threaded brass rod 2 inches long and 1/4 inch diameter. The lower ends of the hard rubber standards are each bored and tapped to receive a screw which fastens them to the base. The electrodes are zinc or brass cylinders 3/4 inch diameter and 5/8 inch long. The upper electrode is adjustable by means of a knurled hard rubber head 2 inches diameter and 1/2 inch thick. The lower one is fastened to a brass plate 2 inches long, 1 inch wide and 1/8 inch thick. One end of the plate is fitted with a binding post. The base is a piece of hard rubber 5 1/2 x 3 inches x 3/4 inch.