600 Lord Rayleigh on the Relation of the 



the diameter 4*5 inches. During the vibration the circular 

 bottom bends, and thus the can must not be held fast round 

 the lower circumference. Support was usually given at the 

 centre only, by means o£ a short length of tube attached 

 with solder. It is possible, however, as will be explained 

 more fully later, to give support at four points of the lower 

 circumference, or rather along two diameters of the base 

 perpendicular to one another. These diameters are at 45° to 

 the line of the electromagnet by which the vibrations are 

 maintained. In order to avoid communicating vibration, the 

 metal handle of the can was attached to a large bung r 

 resting upon a leaden slab, supported in its turn from the 

 floor by a tali retort-stand. Before or after observations the 

 bung could be lifted with the fingers and securitv taken that 

 all the sound heard came direct from the vibrating can. 



The bar electromagnet, by which the iron substance of the 

 can is attracted, lies just below a diameter of the upper rim 

 and is supported from the centre of the base. Since the 

 electromagnet acts as an obstacle to the aerial vibrations in 

 the region where they are strongest, care must be taken that 

 in passing from one can to another geometrical similarity 

 extends to the external form of the magnet and its accessories. 

 The current was supplied by an interrupter-fork and usually 

 both cans under comparison were driven (alternately) from 

 the same fork, so that no question could arise with respect to 

 the accuracy of the musical intervals. In constructing the 

 cans the thickness of the plate employed was taken pro- 

 portional to the other linear dimensions, but this alone would 

 not suffice to secure an accurate tuning. The final adjust 

 ment for the greatest possible response to the intermittent 

 electric current was effected with wax, required only in small 

 quantities. 



Trouble was sometimes experienced from the intrusion of 

 undesired tones. A shunt across the mercury break of the 

 interrupter-fork, as employed by Helmholtz, was useful, and 

 an improvement would often follow- a readjustment of the 

 position of the electromagnet. No observations were taken 

 until false sounds had been rendered entirely subordinate^ 

 if not inaudible. In every case the ear was placed in the 

 plane of the rim opposite the loops L (fig. 1), where the 

 radial motion is greatest, two of which face the poles of 

 the electromagnet. All four positions were utilized, and 

 either the intensity of vibration or the common distance 

 was varied until the average intensity was judged suitable. 

 The intensity aimed at was such that the sound was just 



