OF ARTS AND SCIENCES. 325 



bar. The secondary was of No. 3G wire, wound upon a bobbin, with- 

 half an inch between its flanges, to a depth of about three eighths of an 

 inch, and large enough to slide freely over the half-inch iron rod used 

 in most of the experiments. A galvanometer, of somewhat greater 

 resistance than the secondary coil, was also included in its circuit. 

 The common shaft of the two commutators carried a fly-wlieel and 

 revolution-counter, and was set in motion by a pulley-wheel connected 

 by a band witli a powerful electric motor. 



Though difTerent in detail from the "Differential Interruptor" of 

 Professor Blaserna (as described in Gordon's " Electricity and INIag- 

 netism," vol. i. p. 311 and ff.), this machine, though independently 

 constructed, bears a most striking similarity to that of the Italian 

 professor. The latter needs only the introduction of an iron core, 

 common to the two secondaries, to fit it for the determinations which 

 follow. The former is, however, adapted to tiie use of an alternating, 

 instead of an intermittent, current, which gives for magnetic measure- 

 ments a special advantage, and in determining not the slope of a given 

 portion of the primary wave, but the value of the whole area enclosed 

 by it, would consistently have been called by Professor Blaserna an 

 " Integral Alternator." It being, however, the only machine employed 

 in the phase-determinations which follow, I have alluded to it, when 

 necessary, by its generic name. 



When the shaft is set in rapid rotation, the bar is evidently magnet- 

 ized ; first positively, then negatively, &c. Tiie period of time elaps- 

 ing between two successive maxima of the same sign, in no matter 

 what portion of the bar, evidently agrees with the length of time oc- 

 cupied by a single revolution of the commutator, and ma}^ be called 

 the period of the magnetic wave. Each period repeats, of course, the 

 same continuous series of changes as the one before it. The wave, 

 accord inglj'-, possesses a true phase ; and the difference of phase of 

 the magnetic wave at two different points in the bar may be defined 

 as equal to the arc through which the shaft of the commutator must 

 revolve, after the wave has reached a certain phase of its develop- 

 ment in the one locality, in order that the tardier portion of the same 

 wave in the other locality may attain the same, or rather a correspond- 

 ing phase of its development. It is easy to find four corresponding 

 l)oints in the phase of any two waves ; namely, the two maxima, 

 jiositive and negative, and the two half-way points, or points of zero 

 displacement. By means of one of the latter points the differences 

 of phase were in all cases determined. 



The current induced in the secondary coil would naturally flow one 



