No . 135 The Trusty Servant
Fig . 4 : The discharge tube to produce Xrays .
which pumps a jet of mercury to make intermittent contact with the hanging contacts on the lid ( Figure 7 ). As the impeller rotates , the issuing jets of mercury impinge momentarily on the hanging contacts every revolution , providing the “ make & break ” required . The motor ( Figure 8 ) is started by spinning the rotor by hand , using the wooden knob at the top ; the commutator for the motor being
Fig . 5 : Butt ’ s mercury interrupter .
Fig . 6 : The rotor from the wrecked interrupter .
incorporated into the lid contacts assembly . It is stopped by grasping the wooden knob to stall the motor . Breaking circuits carrying currents of several amperes produces sparks , so , to prevent oxidation of the mercury , an inert atmosphere is required inside the interrupter . This was originally done by purging the interior with coal-gas though , by my time , we used nitrogen from a cylinder .
When I came to the College in 1962 , this early Xray apparatus , updated in the early 1900s , was all still serviceable and was brought out at the ends of terms , for demonstrations of the properties of Xrays . Arthur Lewington , Abley ’ s successor , took great pride in setting it up and instructing the don in its use . He said there had once been two mercury interrupters , but , in the 1930s , a don ( name unspecified ) had failed to purge the air in the mercury pot completely with coal-gas , so that a coal-gas / air explosion occurred , wrecking the mercury interrupter . Only the impeller survived undamaged . This Xray apparatus was last operated in the late 1960s .
Fig . 7 : The principle of Butt ’ s interrupter ( not to scale ).
Rather than a human hand , shapes cut out from sheet lead and other materials , were used as objects to show the properties of X-rays . Health and Safety would ban such demonstrations today . In 2023 , I suspect that I am the last surviving don to have operated this particular piece of historical equipment !
All the pictures are in the College Collections . The two sketches are by the author .
Fig . 8 : The lid of the base containing the mercury , showing the motor and the gas taps for purging the interior of oxygen .
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