468 



Physics. — "Modern electromagnets, especially for surgical and 

 metallurgie practice" By H. du Bois. (Coinmuiiication from 

 the BosscHA-Laboratorj). 



(Communicated in the meeting of May 30, 1914). 



Carrying-Magnets. The lifting-power of the famous steel magnets 

 of Logeman and van Wetteren, has for a long time belonged to the 

 somewhat antiquated subjects. However, traction-electromagnets are 

 now being much more used in different forms, especially for loading 

 and transportation purposes. 



In general we maj say, that for such magnets with armatures at 

 a very small distance Maxwell's well-known law holds; accordingly 

 5378^ measures the carrying-power per unit cross-section of an 

 almost closed magnetic circuit. Prof. Taylor Jones has entirely 

 confirmed this by experiment in the BosscHA-Laboratory. The mag- 

 netic balance as a measuring instrument is equally based upon this 

 fundamental law. Electro-magnetic brakes on this principle are also 

 being more and more used. 



Field-Magnets are of more interest for a variety of scientific and 

 practical purposes. In those Proceedings a description was given of 

 semicircular magnets '), one of which, weighing 350 kg., with an 

 interferrum of 3 X 0,5 mm. gave a uniform field of 59 Kilogauss, 

 while using only a few Kilowatts. With a cryomagnetic "immersion- 

 armature" in a liquid gas at low temperature this reduces to 45 

 Kilogauss. With the heaviest type weighing four times more (1400 kg.) 

 we may cet. par. expect no larger increase than 107o , thus reaching 

 65 and 50 Kilogauss respectively. Until now these field values have 

 not been superseded, no more tlian the Haarlem magnets were. 

 For the investigation of several higlily interesting problems they are 

 absolutely necessary. 



With regard to so small a rise of the field however it requires 

 due consideration whether a further increase of the size of the magnets 

 appears justifiable. For while the weight increases as the third power 

 of the linear dimension, thus becoming quite unwieldy, the field 

 rises logarithmically only, which means a great disadvantage. In fact 

 for a given field we pi-actically obtain only a larger interferrum. 

 However convenient this may be, it is questionable whether it justifies 

 the very high expense which gi-adually begins to surpass an average 

 laboratory-budget. 



*) H. DU Bois, These Proceedings 12 p. 189, 1909; 13 p. 386, 1910. 



