Wave Propagation of Magnetism. 375 



have not been troubled by the vibration of supports or dis- 

 turbances in the room where the instrument was set up. The 

 two telephones can be mounted upon the same support, and 

 the entire instrument can be comprised in a box a foot square. 

 I have used as a source of light a Welsbach burner, which 

 consists of a fine gauze of zirconium placed in the flame of 

 a Bunsen burner. A tin chimney provided with a circular 

 opening of about ^ inch in diameter is placed over a glass 

 chimney and a cylinder of writing-paper over the tin cylinder; 

 a pinhole in the paper at the centre of the orifice in the tin 

 chimney enables one to obtain a point of light on a light 

 ground. This light ground diminishes diffraction-effects and 

 enables one to see the cross wires of an observing-telescope 

 or microscope. With a lime-light or electric light the differ- 

 ence of phase between branch circuits and main circuits 

 through which alternating currents are pulsating can be 

 shown to a large audience. On account of its application to 

 the study of difference of phase in magnetic researches, I have 

 termed the instrument a phasemeter. It is evident that 

 it can be used to study the nodal lines of membranes and 

 plates : for this purpose a plate or membrane provided with 

 a mirror might be placed in front of a movable magnet con- 

 taining at its end a coil of wire, and the vibrations of any 

 membrane or thin plate could be compared with those of the 

 diaphragm of a fixed and standard telephone. 



I have also employed the instrument for studying differ- 

 ences in phase between branch circuits. That there are such 

 differences of phase has been shown by Lord Ray lei gh and 

 others. This portion of my investigation I reserve for a 

 subsequent paper. 



The phasemeter permits of the study of the effect of differ- 

 ent qualities of iron and steel in increasing the self-induction 

 of, and therefore the impedance of, branch circuits ; and it 

 seems to me can be made of great use in the study of alter- 

 nating-current motors. It is well known that two or three 

 electrodynamometers must be employed in the ordinary 

 methods of determining change of phase. The phasemeter 

 can be quickly employed and its adjustments are extremely 

 simple. The application of this instrument to the question 

 of magnetic waves is the subject of this paper. I have 

 employed it both on rings and straight bars, laminated and 

 solid. The ring I employed was 3 feet in diameter, and the 

 diameter of its cross section was -J inch. Two large coils of 

 coarse wire were slipped upon the ring, through which an 

 alternating current could be passed. These coils could be 

 separated or joined together, and by a commutator opposite 



