A Time Switch of Unusual Design

Abstract

About three years ago the author designed and constructed a simple time switch which has answered every need of accuracy and permanency. When set at any desired time, it will continue to give that exposure as many times as the button or foot switch is pressed. When fluoroscopy is used, the timer is disconnected by a small switch, which may be located on any convenient part of the apparatus. With the idea of making radiographs at the time of fluoroscopy becoming more general, a simplified time switch of automatic control is a great help, as the necessity of resetting the small hand timer before each exposure is rather a nuisance when multiple exposures are being made. Of course, there are excellent automatic timers on the market but their price makes a roentgenologist hesitate before putting one on a small machine such as is generally employed for fluoroscopy. The principle involved is that of a solenoid which pulls a core through an oil dashpot. The contact system is extremely simple, and so far as the author knows, has not been employed previously, all of the switches on the market using a much more complicated system. The variable factor in this time switch is the distance which the core moves: the other factors, such as the strength of the magnet and the drag through the oil, remain constant. This feature introduces many elements of simplicity, inasmuch as oil valves are extremely unreliable so far as any accuracy goes. However, the distance of the travel can easily he arranged by simply varying the distance of the contact point mounting from the core. As can be seen by the diagram (Fig. 2), as soon as the button is pressed which energizes the X-ray machine and the time switch solenoid, the core begins to lift upward, carrying with it a piston in the oil dash-pot. When the brass rod touches the upper contact point mechanism this contact is separated from the lower contact which opens the circuit to the X-ray machine, but not to the solenoid. When the operator removes his finger from the control button the core falls back down to its original position. Of course this switch is intended to be used in conjunction with a magnetic relay, such as is found on all modern X-ray machines, but a condenser of two microfarad capacity is shunted across the contact points, which practically eliminates arcing. The contact points themselves need not be of any high-grade material, as those which are used on the old style Ford ignition coil give excellent service. The solenoid is wound with approximately two pounds of No. 28 B & S gauge copper wire and a center tap should be brought out, because if the instrument is to be used on 220 volts the entire winding is used or if on 110 volts only half of the winding is used. The core itself is a piece of seamless steel tubing three-quarters of an inch in diameter, no special precaution having been taken to eliminate eddy currents.