Air Holder No. 3.
Fig. 12 is a cut of a bellows with a contained air holder. It is not practical to try to make this article, as it can be purchased from any plumbing supply house and is not expensive. It is used principally by dentists, but it is also used in laboratories to supply air to the compound blow pipe. It consists of a small bellows held from the floor on iron legs, with a spring inside the bellows to hold them open, and has a rubber bag fastened to the under side to hold a small supply of air. The rubber bag is incased in a string net to prevent it from becoming inflated too much and bursting. This bag serves to equalize the pressure. The size known as No. 10 A will supply 75 cubic feet per hour at a pressure of 1½ pounds to the square inch, which is sufficient for lead as heavy as 24 pounds. For the light weight leads the pressure can be reduced by pumping lightly and not filling the bag more than half full.
Fig. 12.—Air Holder No. 3, Combined with Bellows.
Any of the three described air blast arrangements will answer the purpose, so it is immaterial which is used, and it is left to the discretion of the beginner to obtain whichever is the most convenient.