C. K. Wead—Intensity of Sound. 179 
(10) 2VA=23°94x 10¢h®a? Ergs. = Potential Energy in doth 
prongs 
when the end of the prong is bent a centimeters. 
Ifthe fork is vibrating without giving over-tones, twice in 
h period its energy is wholly potential, and we may use 
equation (10) to compute it. The energy of the vibrating rod 
will actually be a very little greater than of the rod bent into 
the form of the elastic curve having the same maximum ordi- 
nate; for on comparing the curve of a bar fixed at one end 
given by Rayleigh (i, 231) with the elastic curve, we see that 
the latter lies everywhere, except at the ends, a very little nearer 
the line of equilibrium; but the difference in energy is too 
small to be considered here. The error is greater if the prong 
ought to be treated as the free end of a straight rod with two 
nodes, as a comparison of Rayleigh’s Figs. 28 and 31 will 
show; but the latter assumption is certainly further from the 
truth than the former one. 
have used the method given above for finding the energy, 
rather than the one taken.by Rayleigh (i, 202); because for 
experimental purposes it is more convenient to express the 
energy in terms of the amplitude rather than of the radius of 
‘curvature, : 
Scope-tube carrying at its lower end a brass plate bent thus: 
e B_; the distance de was about 80 mm. ; a wire ran from - 
the plate through a telegraph sounder to a battery whose other 
pole was connected to the fork. A series of observations con- 
Sisted in observing the difference of reading of the screw when 
electric contact was made at } and then at ce, usually 1 or 2 mm. ; 
€qual weights were then put in the two pans, 1, 2, 3 and 4kg., 
“and the serew readings again observed; their difference is less 
_ than before by 2a’; at least two determinations were made, and 
the friction of the pulleys eliminated by taking readings when 
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