TCBA Volume 17 - Issue 1
Page 15 of 18
Experimental High-frequency Apparatus
Electrician & Mechanic
Of all the gaps described, none can combine the points of superiority to be found in the revolving gap, illustrated in Figs. 30 and 31. This discharger not only quenches the arc and remains remarkably cool, but it produces a high-pitched spark, which has a distinct and very pleasing musical tone. The gap is no doubt quite familiar to most readers, and several excellent articles on its construction have appeared. As many readers have complained of the difficulty of obtaining the desired results through lack of proper material, and inability to follow the suggested construction, the author offers a few suggestions on the building of a simple but highly successful rotary gap which should be within reach of most readers, as the materials can be obtained at reasonable prices from firms advertising in this magazine.
One of the most important points to consider in the construction of this gap is the rotating disc, 1, of 1/8 in. aluminum. It is imperative that this disc should run very truly on the motor shaft, as the length of the gap may be quite short. The plan suggested for working up the disc will be found practicable and effective.
Referring to Fig. 32, the reader will note that a square of aluminum is centered by means of its diagonals, and a circle having a diameter of 6 in. is struck off. The circumference of this circle is then divided into twelve parts and each point marked with a center punch. A hole is drilled in each corner of the plate so that it may be fastened to a wooden faceplate by means of heavy screws. A 1 in. counterbore will be required for cutting the holes which form the teeth of the gap. Each of the twelve divisions on the plate is to be drilled through with a suitable size of drill to take the tongue of the counterbore, after which the holes may be bored to their finished size. In drilling, or otherwise working aluminum, the metal should be generously lubricated with kerosene oil. After the holes have been cleared, the work should be mounted on the lathe faceplate and the center hole cut to 1 1/2 in. in diameter by means of a very small parting tool. After clearing any burr which may be on the edge of the central hole, the disc may be cut from the square plate on the 6 in. circle. In taking this cut, the feed must be very slow and the greatest of care be taken so that the tool will not catch on the teeth of the disc.
The aluminum disc is mounted in an insulating bushing as shown in the sectional drawing, Fig. 35. The bushing may be of heavy fiber made up in the form of washers, as shown, or preferably turned up from two thick pieces of stock, thereby doing away with the central washer. The aluminum plate is riveted between the fiber discs by means of brass escutcheon pins, and the whole is mounted on a brass bushing which fits the shaft of the driving motor. The final cut on the edge of the aluminum disc should be taken while the brass bushing is mounted on an arbor. This will insure the disc running truly when it is in position on the motor shaft.
The motor may be of the induction type and any of the small power motors on the market will answer. A small fan motor is very satisfactory. The speed of the motor will depend upon the tone it is desired to produce. The average induction motor runs at about 1,800 revolutions per minute on 60-cycle current, and this speed is sufficient unless a very high tone is desired.
The electrodes of the gap may be of round stock if extreme simplicity of construction is demanded, but the flat aluminum electrodes, as shown in the drawings, will be found more efficient. The holders illustrated may present some difficulty in construction, but they represent time and labor well spent. The electrode slides in a brass holder and its movement is controlled by the screw 5, Fig. 30, while a couple of spiral springs secured to pins in the electrodes and to the stock which holds the screw, keep the aluminum back against the screw.
The construction of the holders will be understood more clearly on reference to Figs. 33 and 34, which are drawn to double size. The stock is 1/8 in. brass bar and the upper and lower pieces are made identical with each other. They are joined, either by means of small rivets or by sweat soldering, to the pieces of 1/8 in. square rod, which serve to form a nicely-fitted opening through which the electrodes pass.
The holders are mounted on short pieces of brass rod, 2, in Fig. 30, which are in turn supported by the fiber pillars, 3.
While the present article concludes the constructional features of the high-frequency apparatus, the author hopes to present a few suggestions in the next number on the adjustment and use of the apparatus, together with some hints on the possibilities of the outfit from the lecturer's standpoint.