SCIENCE. 



[X. S. Vol. XXI. Xo. 531. 



proportional to the velocity of the moving 

 system may be used for ballistic magnetic 

 testing. In the Physical Eeview, of March, 

 1903, there is an article by 0. M. Stewart 

 in which it is shown mathematically 

 that such a damped galvanometer con- 

 forms to the ordinary law of the undamped 

 ballistic galvanometer. It is also stated 

 in this article that an ordinary Weston 

 ammeter without its shunt or a \A^eston 

 voltmeter without its series resistance may 

 be used for determining permeability and 

 hysteresis curves by the ballistic method. 



The experiments to be described in this 

 paper were undertaken last spring by two 

 senior students, Mr. E. E. Greve and Mr. 

 A. R. Barkus, under the direction of the 

 writer, for the purpose of comparing the 

 permeability and hysteresis curves ob- 

 tained by means of ordinary Weston in- 

 struments, with the curves obtained by 

 means of a slow-period undamped ballistic 

 galvanometer. It was found that the 

 curves for a laminated iron ring obtained 

 with an ordinary Weston instrument fell 

 about three per cent, below the curve ob- 

 tained with the ballistic galvanometer. It 

 was also found that the permeability curve 

 for a solid iron ring (cross-section 1 sq. in. 

 mean diam. 7 in.) obtained with an or- 

 dinary AVeston instrument fell over ten per 

 cent, below the curve obtained with the bal- 

 listic galvanometer. The cause of this fall- 

 ing below is undoubtedly that the time of 

 the first swing of the ordinary Weston in- 

 strument is too short to take account of ;ill 

 the change in the magnetic flux which oc- 

 curs. The Weston Instrument Co. then 

 eon.structed a special instrument having a 

 greater moinent of inertia and more mag- 

 netic damping than their ordinary instru- 

 ments, and having, therefore, also a mucli 

 longer time for its first swing. This n''W 

 instrument was found to give a permea- 

 bility curve for the laminated ring which 

 agreed exactly with the curve obtained 



with the ballistic galvanometer. For the 

 solid ring the permeability curve was still, 

 however, several per cent, below the curve 

 obtained with the ballistic galvanometer. 

 In order to make a direct reading magnetic 

 flux meter out of this instrument ten yards 

 of flexible cord were taken to be used for a 

 secondary coil, and a resistance was added 

 to the instrument, and this was adjusted 

 so that the instrument would indicate the 

 flux in kilomaxwells changed per turn 

 using this secondary. This instrument 

 was, therefore, called a 'Weston Maxwell 

 meter.' This Maxwell meter has been con- 

 siderably used in the laboratory of 

 Stevens Institute for obtaining magnetiza- 

 tion curves, measuring leakage coefficients, 

 etc., and has been found extremely useful. 

 The Weston Co. are now constructing a 

 second special instrument having a still 

 slower period, which is expected to give 

 accurate results as well for solid iron sam- 

 ples as for laminated ones, and this will 

 be a direct reading, portable and perman- 

 ent Maxwell meter having a uniform scale, 

 which will be generally useful for all kinds 

 of magnetic testing and which can be used 

 directly without previous calibration. 

 Owing to the slow period of these special 

 instruments the extent of their first throws 

 can be very accurately noted. 



Measurement of the Thompson Thermo- 

 electric Effect in Iron. Edwin H. Hall, 

 Harvard Univei*sity. 



Description and Demonstration of the 

 Foulsen Telegraphone. Z. B. Babbitt, 

 New York ; Introduced by Arthur AV. 

 Goodspeed. 



The principles involved in the Poulsen 

 telegraphone and the practical construction 

 of the apparatus were explained. The re- 

 production of human speech was then 

 demonstrated. 



