83 



Tlie collector E. M. F., on the other hand, steadily increases as the capacity 

 •decreases till it reaches a value AcA much greater than the oijen circuit E. M. F. 

 of the machine. A resonant effect comes into play here after the cajjacity of the 

 line neutralizes the inductance of the armature that is very well illustrated by the 

 figure: the line AcA will be a maximum when it passes from A through the center 

 of the circle OAcAoAjO, and will represent the greatest difference of potential 

 that can possibly exist between the brushes so long as R and Kj remain un- 

 <^hanged in value. This rise in potential is due to the current being in advance of 

 the vector OA, for the position of the armature M. M. F. vector is also advanced, 

 and NNc increases the total Hux in the air-gap instead of diminishing it. The 

 cross-magnetizing action of the armature, however, remains approximately the 

 same. 



The introduction of capacity into the line causes the inductance E. M. F. 

 of the armature to move to the position Dc, and the reactance E. M. F. of the 

 ■external circuit to decrease through zero and then increasing, assume a position 

 QcO, considerably in advance of the collector E. M. F., and 90° in advance of 

 the current OBc 



The arrows indicate the relative direction of motion of the vectors as the re- 

 sistance is varied from infinity to zero, or as the reactance is carried from zero 

 capacity to an infinite inductance. 



By following out a similar line of constructions the effects produced by vari- 

 ations of the armature inductance can be studied, and by successfully varying 

 the resistance, inductance, capacity and frequency constants, and constructing 

 corresponding diagrams, a large variety of problems involving the simultaneous 

 variation of several terms can be successfully treated. 



A Method of Measuring Permeability. By A. Wilmer Duff. 



[Abstract.] 



The most common method of measuring the permeability of iron, or the 

 ratio in which the presence of iron strengthens the magnetic field, is to make a 

 ring of the specimen, cover it with two layers of wire, one connected with a source 

 of current to magnetize the ring, the other with a ballistic galvanometer to measure 

 the quantity of electricity induced in this secondary coil by making or breaking 

 the primary current. The galvanometer is calibrated by means of a straight 

 calibrating coil consisting of a non-magnetic core similarly wound with a primary 



