J. Trowbridge — A Phasemeter. 233 



fully tuned to the note to which the diaphragm was to respond. 

 Herr Wien, in a late number of Wiedemann's Annalen, has 

 adopted practically this method of Professor Blake, in what 

 he terms an optical telephone for the measurement of alterna- 

 ting currents.* Herr Wien has added peculiar attachments in 

 order to bring the mirror holder into unison with the note 

 given out by the diaphragm. The following arrangement 

 which I have adopted, although it is less sensitive than the 

 arrangement of Professor Blake and Herr Wien, seems better 

 suited to the instrument which I have termed a phase-meter. 

 Two Bell telephones were provided with diaphragms of three 

 inches in diameter ; these diaphragms were not clamped equally 

 around their edges, but were tuned to the note given out by 

 the alternating current employed by touching the diaphragm 

 at its edges, at suitable points, by means of clamps which could 

 be moved around the edge of the diaphragm until a mirror 

 attached to the diaphragm showed that the latter moved in 

 unison with the current. By turning the telephones around 

 the axis of their magnets the vibrations of the mirrors are 

 adjusted so that they take place in planes at right angles to 

 each other. Thus when one telephone is cut out of the circuit 

 the other gives a straight line of light, indicating that the mirror 

 is moving in unison with the diaphragm. 



As a source of light I have found it convenient to employ 

 the Welsbach burner, which consists of a zirconium gauze 

 mantel placed over the ordinary Bunsen burner. This gives a 

 very steady and intense source of light. A tin chimney is 

 slipped over the glass chimney. In the tin chimney is a cir- 

 cular opening of one half inch in diameter. Over the tin 

 chimney, in turn is slipped, so as to fit it closely, a thin white 

 paper chimney, and a pin hole is made in this paper at the 

 center of the orifice in the tin chimney. This arrangement 

 enables one to see the figures produced by the movement of 

 the two telephones against the cross hairs of the observing tele- 

 scope. Moreover the field of view being thus illuminated the 

 disturbing phenomena of diffraction are almost completely 

 eliminated and a clear bright image of the pin hole is obtained. 



The apparatus can be made very compact, and the two tele- 

 phones with the observing microscope or telescope can be all 

 contained in a box a foot square. With a screen at a distance 

 of twenty feet the figures can be produced of such a size that 

 an audience in the large lecture room of the Jefferson Physi- 

 cal Laboratory can easily perceive them, and note their changes 

 of phase. 



* Das Telephon als optischer Apparat zur Strornmessung. Wied. Ann., xlii, p. 

 593, 1891. 



Am. Jour. Sci— Third Series, Vol. XLIII, No. 255. — March, 1892. 

 15 



