522 
PHYSICS: A. H. PFUND 
of foil soldered with gold, is fused to the lower end of the central glass 
tube (B). The outer surface of the platinum is first tinned and is 
then coated with a thin layer of low melting point solder, such as Wood's 
metal or equal parts of ordinary solder and bismuth. An iron cup (C), 
which fits into the tube D rather snugly, is filled to a depth of about 
5 mm. with this solder. This system must receive the usual baking 
treatment to drive off absorbed gases. In order to effect a seal, the 
lower half of the bulb (D) is immersed in hot water or is gently heated 
with a bunsen burner until the solder melts. Then the cup is raised 
by means of an electro-magnet until the platinum tube is imbedded in 
the solder. The seal is tight as soon as solidification has taken place. 
In order to re-open the seal, the solder is melted and the cup lowered. 
It is obvious that this device may be used to seal off a piece of apparatus 
either temporarily or permanently. 
Experience has shown that a seal of the above type possesses the 
following advantages : 
1. It is tight and vapor free. 2. The openings maybe made large. 
3. Apparatus which has become gassy may be re-exhausted without 
admitting air — as is unavoidable in present practice. 
p 
