|| Recently I realized that today I almost never use a switch
or a tap key when setting up an undergraduate experiment. In the past these
humble items were essential for testing the balance of a Wheatstone bridge,
or reversing the current through a wall galvanometer.
The upper picture shows a tap key that I once used at Kenyon. It is marked "James W. Queen/Maker/ Philadelphia, and so is a survivor from the nineteenth century. It is really a Telegraph Key and was made by Queen of Philadelphia.
The lower picture shows a reversing switch from the Harvard Collection of Historical Apparatus. Although the terminals have screw connections, the contacts are made through mercury cups.
These two switches were bought from Leeds and Northrup by Kenyon College in 1926 when the new Samuel Mather Science Hall was being outfitted. On the right is a double contact switch that cost $15. The charge and discharge key on the right was $22 – both of these figures being obtained from the original invoices. The 1905 catalogue of Electrical Testing Instruments published by L&N notes that the latter “key is admirably adapted to charging and discharging condensers of small or large capacity. The insulation is of the highest, so that no electricity can be lost by leakage. The insulated handle enables the key to be used without insulating the body.” The bulbous design of the insulators increases the surface leakage path length.
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