INDUSTRIAL PROGRESS DURING THE YEAR 1875. lxxi 



turns, which he calls a magnetic proof plane. The coil being 

 attached to a galvanometer, it is to be placed on the required 

 spot, and when the needle is at rest it is to be suddenly re- 

 moved to a distance ; the momentary deflection of the gal- 

 vanometer needle will be proportional to that component of 

 the lines of force at that point which is perpendicular to the 

 plane of the coil. 



Thalen, the Swedish physicist, has written a paper on 

 some experiments which he has made in order to ascertain 

 the location, depth, and magnitude of mines of iron by means 

 of magnetic measurements. By means of careful observa- 

 tions, isodynamic lines are constructed. Then the line which 

 joins the two points of maximum and minimum deviation, 

 or the magnetic meridian of the mine, gives the general di- 

 rection of the ore bed. The intersection of this line with the 

 neutral line indicates the point where it is most desirable to 

 begin mining. Finally, the distance of this latter point from 

 the point on the magnetic meridian of the mine where the 

 deviation is a minimum is one half the distance of the centre 

 of the mass of ore below the soil. 



Beetz has succeeded in producing magnets by electrolysis, 

 the iron having in one case a magnetic moment per gramme 

 of 59, and in another of 214. 



Herwig has observed that the extra-induced currents in 

 iron wires are of remarkable intensity, and supposes it to be 

 due to the transversal demagnetization of the iron. 



Jacques, working in Professor Pickering's laboratory, has 

 made some experiments in answer to Jamin's criticism of 

 Ampere's theory of magnets, in which he shows not only 

 that Jamin's experiments are not themselves fairly capable 

 of such an interpretation, but also that, rightly interpreted, 

 they actually sustain the theory of Ampere. 



Tommasi states the curious fact that if a current of steam, 

 under a pressure of five or six atmospheres, be blown through 

 a copper tube two or three millimeters in diameter coiled in 

 a helix about an iron bar, the bar becomes a magnet, and re- 

 mains magnetized so long as the steam passes. 



Rowland proposes the use of a very small electro-magnet 

 placed upon the stage of the vertical lantern for showing dia- 

 magnetic experiments, and shows by theory that there is no 

 advantage gained by the use of a larger apparatus. 



