306 CARNEGIE INSTITUTION OF WASHINGTON. 



Physical Review for 1915 and 1917, viz, the method of electro-magnetic 

 induction and the magnetometer method : 



(a) Method of electromagnetic induction. Much larger rotors and com- 

 pensators, of pure iron, have been constructed to replace the rotors used in 

 the earlier work, and a very large compensating frame has been constructed 

 and mounted to neutralize the Earth's intensity as nearly as practicable in 

 the regions they occupy. The winding is in two similar coils of parallel wires 

 spaced to produce as nearly uniform a field as possible at the center. With 

 this frame and the uniform field of the building it will be possible to get much 

 more exact compensation than formerly, although fortunately this type of 

 apparatus seems to give results largely independent of eddy currents, which 

 the compensator is designed to eliminate. Provision is made for driving the 

 rotors by electric motor at nearly uniform speeds, and it is hoped to get satis- 

 factory results at higher speeds than formerly. It has not yet been possible 

 to complete a special fluxmeter for this work, and it is going forward with an 

 instrument of the ordinary type, borrowed from the Bureau of Standards. 



(6) Magnetometer method. The apparatus for this method is largely that 

 borrowed from the Ohio State University and is mounted much as in Colum- 

 bus, Oliio, but with great improvements. Additional precautions have been 

 taken against the presence of magnetic matter, against mechanical dis- 

 turbances, and against the possibihty of eddy-current effects on the mag- 

 netometer; many other improvements have also been made. 



The magnetometer is now so mounted that the lower magnet can 

 be placed in either the polar position of Gauss or a position resembling 

 the equatorial position, used alone before. Few observations so far 

 have been made with the magnetometer in the equatorial position. 

 Many observations with iron and cobalt checked by copper to eliminate 

 eddy-current effects, have been made in the polar position. There is 

 already a marked improvement in the quality of the observations 

 over those made earlier, but they have still to be greatly extended and 

 further improved. Satisfactory observations have not yet been made 

 on nickel. (See abstract, p. 313.) 



Rotation by magnetization.- — Since the report of last year, no further 

 experiments have been made on the converse effect to magnetization 

 by rotation, but such preparations for this work have been made as 

 circumstances permitted. A special solenoid has been constructed 

 to magnetize the body in a very uniform field, a compensating frame 

 to neutralize the horizontal intensity of the Earth's field in the region 

 occupied by the body has been constructed and mounted, a photographic 

 device for recording the observations has been nearly completed, 

 and other preparations have been made. It is hoped that observations 

 will soon be well under way. 



In one form of this investigation the magnetizing solenoid is wound 

 on the core to be magnetized, and the other solenoid is used to neutral- 

 ize the Earth's vertical intensity. An*angements have been made to 

 perform the experiment in this manner also, and a rotating commu- 

 tator to fm'nish a "rectangular" alternating current of constant fre- 

 quency has been constructed. 



