252 Mendenhall and Waidner — 



that the field of view includes the space between the coils, and 

 shows the needle-system clearly defined against the illuminated 

 milk-glass ; when the system hangs in the desired position par- 

 allel to the coil-faces, the magnets appear end-on, and they are 

 so thin that a very slight departure from this end-on position 

 can be detected. In fact, "finding the spot of light" becomes 

 easy ; it can be adjusted to within a few centimeters of its 

 desired position on the scale by adjusting the control magnets 

 until the system- magnets appear end-on, after which the care- 

 ful adjustment of zero can proceed as usual. The dimensions 

 of the coils being small, very light systems can be used, the 

 control field can be made very nearly equal at the upper and 

 lower coils, and magnetic shielding can be more easily applied. 

 Construction is simplified by the circular form of the case, per- 

 mitting the extensive use of lathe work. 



We would suggest the change of the top of the fiber tube 

 from its present form to that given in fig. 1 ; the centering 

 screws there shown would allow considerable horizontal 

 adjustment of the fiber, and would clamp it in any desired 

 position. A larger but shorter fiber tube — say 2 cm internal 

 diameter, a heavier base, and a method of clamping the outer 

 case above the base, so as to permit free access to the interior, 

 would also be better. 



In connection with this instrument we have used a support 

 for control magnets shown in plan in fig. 2, and in perspective 

 in fig. 3 ; this permits an independent rotation and vertical 

 motion of the symmetrical pair of control magnets, by the 

 observer at the scale, using the long shafts shown in fig. 2. 

 Independent changes in direction and magnitude of the con- 

 trol vector are thus possible.* In our apparatus, one turn of 

 the observer's handle rotates the control magnets through 20°, 

 and this is sufficiently delicate for most circumstances. It is 

 convenient to have the magnetic force due to the control mag- 

 nets nearly the same at the upper and lower coils, because the 

 behavior of the needle can then be more easily predicted ; 

 with our control magnets 35 cm above the coils, there is a differ- 

 ence of about 15 per cent, which is ordinarily not enough to 

 confuse matters. For example, if (fig. 4) AO represents in 

 magnitude and direction the earth's magnetic force, AB the 

 direction in which it is desired to have the resultant field, then 

 for a given height of the control magnets there are two posi- 

 tions of the magnets, OC and OD, which give a resultant in the 

 desired direction ; one, OC, giving a long period of vibration, 

 the other a short period. These two periods will be obtained 

 provided the force OC due to the control magnets is greater than 



*See Waidner, chap, on galvanometers, Ames & Bliss, Laboratory Manual; 

 also, Rogers, in Nichols, " The Galvanometer." 





