56 Messrs. Boys, Briscoe, and Watson on the 



We have made use of Joule's instrument, not exactly in the 

 original form, but with two classes of index, and with an 

 essential improvement which makes it extraordinarily sen- 

 sitive without greatly affecting its simplicity. The first class 

 of index depends for its working on the fact that when a small 

 piece of phosphorus is placed in air it gradually oxidizes, and 

 the oxide formed slowly falls, if the air is still, in a well-defined 

 column, appearing like smoke. If there is any very small 

 motion of the surrounding air, this column of phosphorous 

 oxide will be deflected. It thus forms an excessively de- 

 licate and light index for detecting movements in the 

 air. 



To overcome the difficulty of keeping the index at rest, 

 owing to radiation from neighbouring bodies or to draughts 

 of varying temperature, we have surrounded the divided tube 

 by another, which was kept rotating continuously by clock- 

 work. If any part of this guard-tube becomes slightly heated, 

 it is carried round and made to heat the instrument equally all 

 round, and so it produces no effect. 



As the determination on theoretical grounds of the most 

 suitable dimensions presents considerable difficulties, we 

 decided to make a series of tubes of different sizes and 

 compare their sensibility. The object of the instrument 

 being to measure the heat developed by the electromagnetic 

 radiation in a wire within the instrument, it was necessary 

 to make both the tube and guard-tube of some insulating 

 material, such as glass. 



The tube to be used was cleaned, and a roll of brown paper 

 about 1 inch long was pasted round one end. This fitted 

 tightly into a mahogany ring, D (Plate V. fig. 5). The 

 tube was divided longitudinally by a partition of card- 

 board (F), which was made to fit tightly into the tube. This 

 partition extended from the bottom to within about an inch 

 of the top, which end was closed by a cork. 



The projecting end of the mahogany ring (D) fitted into a 

 corresponding hole in a piece of wood (E), which was itself 

 supported on two blocks (A and B). The tube (represented 

 by the dotted circles in the plan, fig. 6) came directly over 

 this space between A and B, the partition being parallel to 

 their adjacent faces. 



Between A B, and immediately beneath the tube, two glass 

 semi-cylinders were placed, with their convex surfaces facing 

 each other. They are shown in plan in fig. 6 (MN), and 

 one is shown in perspective in fig. 7. At 2 centim. from the 

 top of one of these semi-cylinders a small paper scale (fig. 5), 

 divided in half-mill imetres,, was attached. When a current 



