PHYSICS. 491 



that steel submitted to great pressure, and cooled under tliis pressure, 

 possesses coercitive force, and may be permanently magnetized. More- 

 over, steel thus treated maybe heated, and even forged, without losing 

 this property. Instead of being ephemeral and transitory, as coercitive 

 force is in steel hardened in a bath, that produced by compression is 

 permanent, unalterable. In proof of this, the author broke in pieces 

 the magnets of a magneto-electric machine, forged them into a bar, 

 compressed this bar, reformed the magnets, remagnetized them, and 

 found them to have, on testing, the same magnetic torce as before. 

 {Comptes Eendus, October, xcv, p. 587.) 



Goolden has contrived a convenient form of diji circle, which is con- 

 structed by C. Casella, of London, and which is sufficiently accurate 

 for class Instruction. {Nature, December,' xxvii, p. 127.) 



Kohlrausch has described some portable instruments for measuring 

 variations in the intensity of terrestrial magnetism. They include (1) 

 a bifilar magnetometer, in which the magnet is inclosed in a rectangular 

 box 12.5 X 5 X 5*=™, made of copper and having a glass front. The cover 

 carries a tube 2.5cm j,, diameter and 25«"" high, furnished with a torsion 

 circle, supporting a bifilar suspension consisting of two brass wires 

 0.05""" in diameter, 8"°^ apart. The magnet is lO'^'" long, 1.4«'" in diam- 

 eter, with a hole 9°^™ in diameter through the axis. It weighs 83 grams, 

 and is supported by a stirrup which carries a mirror. (2) An instrument 

 for measuring variations by means of directing magnets, consisting of 

 a compass, the needle of which is maintained at 90° to the magnetic 

 meridian by four magnets carried by a metallic circle, with which is a 

 second circle graduated. The author claims that these instruments 

 will give the variations to 1-lOOOOth of their total value. {Wied. Ami., 

 V, XV, p. 550 ; J. Phys., October, II, i, p. 467.) 



2. Eleotromotors. 



The theory proposed by Exner, that electrification does not result 

 from contact of metals, but from the chemical action of the air, produc- 

 ing oxidation, the layerof oxide remaining there i)ermanently, has been 

 criticised by several physicists. Exner's experiments tended to prove, 

 as he thought, that the superior limit of the difference of i)otential of 

 the metal and its oxide is proportional to the heat of oxidation of the 

 metal. In contact with platinum, which he supposes incapable of ox- 

 idizing, the other metal with its layer of electrified oxide forms a con- 

 denser, the charge of which, measured by an electrometer, will be one 

 half the electromotive force belonging to the metal in question. Stole- 

 tow has taken up in order the eleven experiments which Exner cites in 

 his second memoir as contradictory to the contact theory as ordinarily 

 admitted, and shows that, considering the earth as a metallic conductor, 

 all the eleven may be very simply explained in conformity with the 

 contact theory. Sokoloff has examined mathematically the condenser 



