1G ARMATURE CONSTRUCTION 



the specimen and the magnet poles. In the instrument, as shown, 

 the magnetic density in the samples will be about 4000 C.G.S. lines. 



The damping of the swing of the magnet is produced by a dash- 

 pot consisting of a little vane, which moves in a tank of oil. The 

 degree of sensitiveness of the instrument is adjusted by raising or 

 lowering the nut, g, on the lower side of the magnet; the centre 

 of gravity of the oscillating system is thereby raised or lowered. 

 The pointer is set to zero at the middle of the scale by means of 

 the levelling screw, h, and also by means of a nut, which runs on 

 a screw projecting laterally from the middle of the magnet. Since 

 the average of the readings on both sides is taken, it is unnecessary 

 to have the zero in exact adjustment ; but it should be set near 

 enough to prevent the scale readings on the two sides from being 

 very different. 



The instrument is also supplied witli an arrangement for lifting 

 the movable system from its bearing when not in use. 



The tests should be made from pieces cut at random from differ- 

 ent parts of the sheets whose quality it is desired to determine. 



Large sheets may be tested in bulk for hysteresis and perme- 

 ability by means of an apparatus which the Siemens-Schuckert 

 Werke, of Vienna, have placed upon the market. The sheets to be 

 tested are slid, by suitable means, into a large cylindrical drum so 

 that they constitute a ring of about a meter diameter, with a 

 section measuring a meter or more in one direction by a couple of 

 millimeters or so in the other direction. This apparatus may be 

 seen at the right in Fig. 15. 



The frame is provided with a permanent annular winding 

 enclosing the sheets, and from four to six sheets are thus generally 

 tested together. The method permits of readily unloading and 

 testing a few sheets from a given shipment, in order to determine 

 whether or no the entire shipment shall be accepted. 1 



1 For description of this apparatus, see an article by Richter, entitled 

 " Eisenprufapparat fur gauze Blechtafeln" (Elektrotechn. Zeiischr., 24, pp. 341- 

 343, 7th May 1903). 



The Ewing apparatus and the Siemens-Schuckert apparatus, as well as a 

 number of other methods of testing sheet iron, are described in considerable 

 detail in the first chapter of Electric Machine Design (a revised and enlarged 

 edition of Electric Generators), London, Engineering, 1906. 



On p. 187 of the Elektrotechn. Zeiischr., for 5th March 1903, Brion describes a 

 development from Richter's method. Brion's method permits of testing the sheets 

 in the flat state, two sets of sheets being employed, and the ends joined by strips 

 of such small length and large cross section that the loss in them may be ignored, 

 or an approximate correction may be employed for the loss in the pmall strip. 



