80 



The Velocity of Sound. 



By Hubert Painter, B.Sc, F.C.S. 

 (Read before the Physical Section, June 12th, 1917.) 

 * 



The velocity of sound in air can be calculated with great 

 accuracy from theoretical considerations and it has frequently 

 been measured experimentally. The classic experiments essenti- 

 ally consist in determining by some form of chronograph the time 

 taken by a sound, generally the report of a cannon, in travelling 

 over a carefully measured distance of several miles. Thinking it 

 desirable to have a method of making a direct measurement (in- 

 direct methods are already in use) that could be easily carried out 

 in any building possessing a long corridor, I have devised the 

 simple apparatus now before us. 



It consists of a pair of small electro-magnets, bar-shaped, 

 each fastened to a strip of wood. These wooden strips can be 

 clamped at any desired height, each on one of a pair of upright 

 stands about 4ft. high, and, when so clamped, the pole faoes of the 

 magnets are horizontal. The stands are placed on tables at the 

 ends of a long corridor or passage or out-of-doors in a quiet spot. 

 An electric circuit is arranged, including the two electro-magnets, 

 in series, one or two dry cells and a switch. Two small steel 

 balls are also required. Clamp the magnets so that they are at 

 the same height above the tables, switch on the current, and put 

 a ball on the downward-facing pole of each magnet. If two or 

 more ** listeners " take up a position behind one of the stands 



