510 



Mr. W. H. Preece. On the Conversion of [Mar. 10, 



of a disk exposed to the source of energy, be either blackened by lamp- 

 black or camphor carbon, or if it be polished or whitened. 



An apparatus was constructed similar in principle to that described 

 by Messrs. Bell and Tainter. The source of light (L) was an oxy- 

 hydrogen lime-light. The rotating disk (B,) was of zinc perforated 

 with holes, which could be noiselessly rotated, so as to obtain 1,000 

 intermissions per second. Grlass lenses (Gr) were employed to focus 

 the light upon the perforations of the rotating disk, and another (Gr'), 

 to render the rays parallel on the other side of the disk. A mahogany 



case or cup (C) to retain the disks to be experimented upon was 

 constructed as shown in section in fig. 3, and fixed 400 centims. 

 from the source L ; a, being the disk 5 centims. in diameter, 

 clamped on by screws ; b, a brass tube, to which the india-rubber 

 hearing tube Qi) was fixed; c, a circular air space behind the disk 

 6 centims. in diameter, and 3 millims. to 5 millims deep. Cavities of 

 various dimensions and forms, spherical, conical, and trumpet-shaped, 

 were tried, but the ones described were those which gave the best 

 effects. 



Experiment 2. — An ebonite disk well blackened on one side when 

 exposed to the intermittent rays was found to produce sounds, which 

 Professor Hughes estimated to be about 20 as compared with his 

 sonometer scale.* 



* Professor Hughes' Sonometer (" Proc. Koy. Soc," vol., 29, p. 56) proves an 

 excellent apparatus to estimate the relative intensity of low sounds, for by shifting 

 the induction coil from one side to the other, we can pass from an absolute zero, or 

 pure silence, to a limit of sound of considerable intensity. The scale divides this 

 space passed over into 100 equal parts, any number of which give a fairly approxi- 

 mate value of the sounds compared. The ear has to determine the equality of the 



