Alternating Current Magnetic Curve Tracer. 107 



This condition is not fulfilled by any magnetic-curve tracer, 

 so far as I know, that has ever been described. The failure 

 being due to mutual interference (mechanically or magneti- 

 cally) of the separate displacements of the indicating system, 

 to lack of proportionality of these two displacements to the 

 intensity of magnetization and magnetizing force respectively, 

 or to the inability to interpret these displacements in terms of 

 Absolute Units. 



In the following description of a continuous and alternating 

 current magnetic-curve tracer, the order observed will be the 

 actual order of development from the observation of the 

 underlying principle to the embodiment of this principle in 

 the present form of instrument. 



§ 2. Several years ago (in the early part of 1892), while 

 making an investigation of the losses in iron magnetized by 

 alternating currents, the w T riter used, he believes for the first 

 time, an instrument which may be conveniently called an 

 alternating-current magnetometer. A small helix of fine 

 insulated wire was suspended bifilarly, so that the axis of the 

 helix was in the earth's magnetic meridian. The lower ends 

 of the bifilar wires were put in connexion with the free ends 

 of the helix, and the upper ends of the bifilar wires were 

 arranged so that the suspended helix might be made to form 

 part of a circuit, composed of a few cells of secondary battery, 

 a regulating resistance, and a revolving contact-maker carried 

 on the shaft of an alternating-current generator. It is clear 

 that when the armature of the alternating-current generator 

 was set in rotation at a constant angular velocity, an inter- 

 mittent current of constant mean intensity and of definite 

 period and phase relation was passed through the suspended 

 helix. It was observed, on bringing a long rod magnetized 

 by an alternating current from the generator whose armature 

 shaft carried the contact-maker, that the suspended helix was 

 deflected from its position of rest through a definite angle for 

 any definite position of the rod ; and that for any fixed 

 position of the rod, by varying the point in the revolution of 

 the armature-shaft at which the intermittent current was 

 made through the suspended helix, a series of deflexions of 

 the suspended helix to the right and left of the zero position 

 was obtained. Thus, by means of the suspended helix carry- 

 ing an intermittent current of proper periodic time and of 

 constant mean intensity we are enabled to measure directly 

 the magnetic state of the iron at any definite point of the 

 alternating-current waves. The alternating-current mag- 

 netometer method, employed to measure the losses in iron 

 magnetized by alternating currents, has been described in the 



