:U8 Dr. S. R. :Milner on an 



this, and also of the modified form of pump-head adopted, 

 will be best made clear by considering the whole action of 

 the pump when set into operation. The modification in the 

 form of pump-head, as is seen by a reference to the figure, 

 consists in the entrance of ihe capillary del into the side 

 instead of the bottom of the auxiliary chamber, while this 

 latter is connected below by a tube//' containing a mercury- 

 trap F, with the tube hi/ leading from the reservoir to the 

 pump-bulb. 



Method of Action. 



To set the pump in action the lever of the three-way tap 

 is turned down on the left-hand side so as to connect u to r, 

 and a preliminary exhaustion to about 10 cms. of mercury 

 pressure is given by the water-pump to the vessel to be 

 evacuated, to B, D, E E', and the air space of A simul- 

 taneously. The stopcock M is then turned off and the 

 battery contact P closed, when the pumping will go on 

 automatically. 



A contact between the wires U and Y in the reservoir is 

 already made by the mercury there, so that a current flows 

 round the circuit PUVK/i. (The electrical connexions 

 are indicated by the dotted lines in the diagram.) Thus the 

 tap is turned so as to connect the air space of A to the atmo- 

 sphere ; at the same time the lever / 1' breaks the circuit at 

 Ji, and prevents the sparking inside the reservoir which 

 would otherwise occur as the contact U V is broken by the 

 mercury leading the reservoir. The speed of the mercury 

 as it passes into the pump-bulb — and also up the side tube 

 f'f as far as the point / where it is stopped by the mercury 

 trap there — can be adjusted to have any value by a screw 

 clip on a rubber tube connected to z, which regulates the 

 rate of admission of air. 



Meanwhile, there being air at a pressure of a few centi- 

 metres in D, the mercury of the gauge is pushed up in the 

 tube g so as to make contact between the terminals Y Y', and, 

 the contact/ i of the tap-coil being now also made, a current 

 passes round the circuit P IT Y Y' Br K'/ i. By a suitable 

 resistance R inserted in series with the coil B?', this current 

 is made small enough to keep the contact Q broken without 

 turning the tap T. Thus the latter is not turned until the 

 mercury from the pump has expelled the air into E and 

 reached the electrode Z, thus completing the circuit PUZK'/ i. 

 This turning puts the reservoir in connexion with the water- 

 pump, and again immediately breaks the circuit of the 

 battery at /' i outside the pump. The mercury sinks in 



