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PHYSICS: A. G. WEBSTER 
placement of 1/24,000 millimeter of the vibrating disk, which is of the same 
order as could be used with the interferometer, and far better than could be 
done with a microscope. This phonometer is as sensitive as the normal ear 
(but for a very Hmited range). 
2. Phone -The phone, after having gone through many different forms, has 
now assumed a form closely connected with the phonometer, particularly as 
to the possibility of tuning. Fig. 3 shows the form now chiefly used. The 
disk 3, placed centrally in the hole of the resonator, supported in its wires and 
tuning pins 7, is actuated by an electromagnet 9, the current of which is inter- 
FIG. 2b 
rupted by a separate electrically-maintained tuning-fork with a platinum 
point dipping into mercury. If exact purity of sound is not important, and 
the slight fizzing always associated with a dry contact is unobjectionable, a 
spring interrupter may be used, and the phone is self-contained, but generally 
it is better to have a set of forks to be used in the interchangeable stand, to 
control the phone at various pitches. 
During the last year I have made the acquaintance of the pliotron and 
found it to be a wonderfully convenient instrument for the production of 
