

456 R. W. Willson — Magnetic Field in 



Art. LIY. — The Magnetic Field in the Jefferson Physical 

 Laboratory. Part II ; by K. W. Willson. 



In the February number of this Journal I have given an 

 account of some observations made upon the variations of the 

 Horizontal Intensity in different parts of the Jefferson Phys- 

 ical Laboratory in 1886-7, and upon the disturbance in the 

 magnetic field produced by the presence of iron steam pipes 

 and other iron masses. I have recently received a copy of the 

 dissertation of Dr. Gustav Pasch of Aix-la-Chapelle* giving 

 the results of similar observations upon the effect of the heat- 

 ing apparatus in the Physical Laboratory at Wiirzburg and 

 of passing railway trains, etc. 



During the past summer an iron water-pipe six inches in 

 diameter was laid for some hundreds of feet, nearly parallel 

 with the west wall of the Jefferson Laboratory and passing it 

 at a distance of about twenty feet. The question was raised 

 whether this would produce any serious change in the magnetic 

 conditions previously observed in the building, and at the re- 

 quest of the Director, Professor Trowbridge, I have made a 

 re-examination of the variations of the Horizontal Intensity at 

 the points observed in 1887. A further interest in the results 

 lies in the fact that since the former observations were made 

 both the stove and iron table top, to which I attributed certain 

 disturbances, have been removed, and it may now be seen 

 whether the disturbances have disappeared with their assumed 

 causes. Whether there has been any change in the absolute 

 value of H, about O170 C. G. S. units, I have not attempted to 

 determine. 



Pooms 12 and 17, were not included in the observations on 

 account of the great variations caused by the proximity of the 

 steam-pipes at A, B, C, F, near the east wall (fig. 4). 



The same instrument was used and mounted upon the 

 same stand, a small high table carrying the variometer, five 

 feet from the floor and resting on three very easy casters. 

 Attached to the stand at about two and a half feet from the 

 floor is a magnetometer with mirror and scale which serves" for 

 the rapid orientation of the stand at each new station. The 

 distance between the variometer and magnetometer is such 

 that neither has any appreciable influence upon the readings 

 of the other, and at no point of observation was their differ- 

 ence of meridian (from local disturbances, such as proximity 

 of the magnetometer to the piers) sufficient to cause an error 

 so great as -^fo-Q in the concluded value of H. 



*Magnetische Untersuchungen. Wiirzburg, 1888. 



