^^iilii- 



i\. 



VARIATIONS IN THE EARTH'S MAGNETIC 



FIELD.* 



Feancis E. Niphek. 



On account of the possibility of local disturbance due 

 to street cars, it was thought desirable to repeat some of *-'-^'7a;?Y 



the work described in a former paper in these Trans- '"^^' ^ork 



actions, No. 2, Vol. XXII. This was done during the ti."^.^^^"^^ 

 summer of 1913 in a large tent 18x20 ft. located on 

 the lake shore at the north end of Lake Huron. The 

 station was about 50 ft. from the lake and about half a 

 mile N. W. of the village of Hessel, in Mackinac County. 



The magnetic needle was a piece of knitting wire 7 

 cm. in length, suspended upon a fiber of unspun silk 

 about 40 cm. in length. The enclosing case was formed 

 from a large glass bottle, the top of which was removed. 

 A metal cap fitting closely around the top of the glass 

 jar thus formed, was provided with a vertical brass tube, 

 having at its top a torsion head and means for attach- 

 ing the suspension fiber. This metal cap was sealed to 

 the glass jar by means of adhesive rubber tape. 

 The jar was mounted in a closely fitting base provided 

 with leveling screws, which were about 30 cm. apart, in 

 order to secure stability. A fine copper wire soldered 

 to the middle of the needle, served as a means for at- 

 taching the suspension fiber. It also extended below 

 the needle, and to its lower end was attached a horizontal 

 wire about 6 cm. in length, which dipped into coal oil 

 in the bottom of the jar, and served as a damper for the 

 needle. Attached to the wire suspension of the needle 

 was a small mirror, by means of which the motion of 

 the needle was observed, in the usual way with a tele- 



*Presented before The Academy of Science of St. Louis, October 20, 

 1913. 



(109) 



