170 McCLELLAX— USE OF OSCILLOGRAPH. [April 14, 



the mirrors have practically the same horizontal axis, though they 

 are not in the same vertical plane, necessarily, the spots when 

 focussed on the plate will be in the same horizontal line. They 

 must be adjusted, however, until they have the same vertical axis 

 when not vibrating. This adjustment, as well as the focussing, is 

 done by means of a small glass cylinder with a ground glass end. 

 This is entered through the back of the chute, and is of such a 

 length that the ground glass' is in the plane of the falling plate. 

 The focussing is finished and the cylinder removed. After a time, 

 that is with some experience, the focussing can be done from the 

 front of the box. There is no adjustment for the verticality of the 

 mirrors. The coils may be twisted, however, so that they may be 

 brought to various horizontal positions by adjusting screws on the 

 side of the standard. 



The double oscillograph permits the simultaneous taking of two 

 independent curves. Since the loops are so fine, and the area 

 of the field so large, comparatively, the loops move in a constant 

 field. The amplitude of the wave is therefore proportional to the 

 maximum value of the current passing through the loops. This 

 will be so only when the damping is critical, that is, sufficient to 

 prevent running past the static position for the same current, and 

 not too much to prevent the loop reaching its static position. A 

 proper adjustment of a non-inductive resistance in series with the 

 loops would make the deflection a definite fraction of an ampere 

 per millimeter. One of the suggested uses to which the double in- 

 strument may be put is to obtain the current and potential differ- 

 ence curves for the same piece of apparatus. That this cannot be 

 done, at least exactly, will be apparent from a little study of the 

 conditions. The problem is similar to the wattmeter problem, in 

 which there is always a slight error, due to either the current or the 

 E.M.F. required for one of the coils. The inference is also fre- 

 quently suggested that the difference in phase, as indicated by the 

 record of the two curves, is the true difference of phase between 

 the quantities. That this is never true, and seldom approximately 

 so, will be apparent from the following discussion : 



The simplest method of putting the oscillograph in circuit is 

 shown in Fig. 3. L is the apparatus for which the current and 

 E. M. F. curves are to be determined. O x and 2 are the two loops of 

 the oscillograph joined in circuit as shown. Usually it would be 



