473 



differences tlian of the strengtli of currents, and therefore, to take 

 a small resistance for the dynamo-coil in comparison to that of the 

 current meter. On the other hand it is desirable (o take a resistance 

 for the djnamo coil of at least some limes ten Ohm, in order to 

 exclude a too great influence of transit-resistances. 



The following can be recorded about the prepaiatory experiments 

 carried out with this instrument. 



One of the specimens of the scansimeter showed the following 

 unexpected effect, which the others showed less or did not show 

 at all. After the axis of rotation had been carefully placed horizon- 

 tally in the laboratory, and the position of equilibrium of the uiagnet 

 had been regulated in such a way that a rotation of the axis did 

 not generate any current, a current could be produced arbitrarily 

 in both directions by changes in the position of the brushes only. 

 This phenomenon, ratlier startling at first sight, can be explained 

 as follows ^). 



Evidently the magnetic field cannot be considered as resulting 

 from two fields, one after the axis and another pei-pendicular to 

 the axis, but here a third component has to be accepted, which 

 we can assume to be horizontal and perpendicular to the axis. If 

 the brushes are fixed in such a position, that the com|)onent in the 

 vertical plane comes to ful development, i. e. that the commutating 

 of the current takes place at the moments the difference of potential 

 is zero (fig. 6), the effect of the horizontal component vertical to 

 the axis is likewise zero, because for this component the commutation 

 comes at the moments the B. M. F. has its maximal value (fig. 7). 

 When the windings are distorted with respect to the collector or 

 the brushes with regard to the magnetic field, then this horizontal 

 component is no longer inactive, so that a current can be produced, 

 at will in either direction (fig. 8 and 9). 



The wryness in the field, which this specimen showed, can easily 

 occur, when the centering of the armature between the poles of the 

 magnet resp. the poleshoes, as well sideways as in the dii'ection 

 of the axis, is deficient, which can very well be the case in an 

 air gap of not more than some tenths of a millimeter. This 

 explanation requires, as was confirmed by the experiment, that, 

 when the relative position of collector and coil is the right one, such 



') We may neglect the armature reaction because it was very small, given the 

 weakness of the currents. Hesides, by taking into account this reaction hardly 

 anything changes in the reasoning. 



