DOUBLE-POLE DOUBLE-THROW MERCURY SWITCH BY GLENN H. DAMON
In many industrial and research problems electrical contact through a knife-edge switch is not satisfactory, due to irregular contact resistance. Where extremely small currents are to be accurately measured, this variation in resistance may have an appreciable effect on the results obtained.
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The use of a mercury switch eliminates to a large degree this variation in electrical contact resistance. It is, however, difficult to obtain suitable mercury switches for certain types of work. Satisfactory single-pole singlethrow switches may be obtained from several companies manufacturing such electrical apparatus. These switches are generally furnished unmounted and being made of glass are readily broken. They are not easily adapted to problems requiring single-pole double-throw, double-pole double-throw, or re versing switches. The switch shown in the accompanying diagram eliminates the difficulties attendant to the use of the usual knife type switch. This switch was developed to perform all of the functions of a knife-edge switch of the double-pole double-throw type. I n addition, it has the advantage of a constant contact resistance. The device is constructed so that it may be made to operate automatically by the addition of suitable electro-magnets and relays. The
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switcli may also be made reversing by changing the connections between the electrodes and the binding posts as in the case of the double-pole doublethrow knife switch. The body of the switch is made of an insulating material such as bakelite, the contacting chambers being either molded or drilled in the insulating material. The central portion of each contacting chamber is depressed from the horizontal so as to provide a neutral or no contact position when the switch proper is parallel with its base. The electrodes are any material which will make good electrical contact with mercury. Iron, nickel and copper have proven the most satisfactory. The electrodes are sealed in the bakelite plugs (A) with either bakelite or de Khotinsky cement. These plugs, with the electrodes in position, are either screwed or sealed into the positions shown. The entire apparatus is mounted on an insulated base as shown in the drawing. The main part of the switch is balanced on an axis (B) which can be readily turned by a knurled knob (C). By proper adjustment, the switch will stay in any desired position until changed by the operator. Automatic control may be accomplished by the proper electromagnetic system. The size of the switch may be varied over a considerable range without changing the essential features. It can readily be seen that this switch may be designed for small or relatively large currents. All glass parts have been eliminated, consequently the switch is quite rugged. The neutral or no contact position is a unique and essential part of this switch. An investigation of the literature has revealed no mercury contacting device of either single or double throw type which has a neutral position. Michigan College of Manang and Technology, Houghton, Machagan.
