322 



MAGNETIC METHOD 



[Chap. 8 



American Askania Corp. 



Fig. 8-18a. Section and scale 

 of Askania-Schmidt vertical 

 magnetometer. 



uated tube for deflector 



newer models, stops are provided for 



180° and 90° positions. 



Formulas for normal operating conditions 

 are readily derived from eq. (8-18) by writ- 

 ing tan 277 = (s - So)/f , where So is the reading 

 corresponding to t; = (20 in the older, 30 

 in the new telescope), s is the reading 

 corresponding to t), and f the focal length of 

 the objective lens. Neglecting tan^ 77 com- 

 pared with 1, tan 7; = (s - So)/2f sc 

 that 



netometer should be read, a magnet is placed 

 under the instrument, south pole up. If this 

 causes the readings to increase, the scale is 

 being read in the correct direction. 



The magnetic system (earlier type) (see 

 Fig. 8-19) consists of two magnetized bars 

 (tungsten or cobalt steel) attached to an al- 

 uminum cube, which carries the knife edge, 

 the mirror, two lateral screws, and one verti- 

 cal screw. The two lateral screws, provided 

 with counter screws, are for latitude adjust- 

 ment ; the vertical screw is for the scale-value 

 adjustment. In the newer models tempera- 

 ture-compensated magnetic systems with 

 steel frame, compensating aluminum spindle, 

 and invar latitude spindle are used. If large 

 scale values are desired, the brass screw may 

 })e replaced by a gold screw. 



The top of the tripod is graduated and may 

 be rotated about a vertical axis. A detach- 

 able compass 



is supplied ^ ^ft^ ; 



with the in- 

 strument for 

 orienting it 

 into the mag- 

 netic prime 

 vertical. Be- 

 low the tripod 

 head is a grad- 

 magnets. In the 

 the 



American Aakania Corp. 



Fig. 8-186. Simplified Askania 

 magnetometer. 



