PHYSICS. 239 



Deprez lias contrived a new velocity-re^nlator for electric motors. It 

 consists essentially of an elastic spring, fastened at one of its extremi- 

 ties to the rotating axis and parallel to it. To the other end is attached 

 a small mass of brass traversed by a screw by which its distance from 

 the axis may be adjusted, and the tension of the spring at the same time 

 increased. The current which drives the motor passes through this 

 spring. If now the rotation is so rapid that the free end of the spring 

 is thrown away from the axis by the centrifugal force, the contact is 

 broken and the current is interrupted ; the speed of the motor is dimin- 

 ished, contact is renewed, and so on, the velocity being exactly regu- 

 lated by the adjusting screw.— (rJ. Phi/s.^ viii, 10, January, 1870.) Fou- 

 vielle has devised a form of gyroscope whose rotation is produced by 

 magneto-electric currents. A tiat coil of wire, like that used in galvan- 

 ometers, has an intermittent current sent into it from an induction coil. 

 When a star-shaped piece of iron, mounted on a pivot so as to rotate 

 like a compass-needle, is placed in the center of the coil, it begins to re- 

 volve whenever a horse-shoe magnet is placed above it vertically, or 

 when a bar magnet is placed in certain directions with reference to it. 

 Keversing the direction of the current reverses the direction of the rota- 

 tion. — {Nature J xxi, 593, April, 1880.) 



2. Of Liquids. 



Eudorflf has described a special form of volumenometer used by him 

 for the determination of the specific gravity of powders. The new ap- 

 paratus is composed entirely of glass, and consists of a cylindrical reser- 

 voir of 25<'<' capacity, from the lower part of which a tube passes, this tube 

 being enlarged just below the reservoir, and having a cock at its lower 

 end. The reservoir is closed at top by a ground stopper furnished with 

 a stopcock, and to it laterally is attached a long U-shaped manometer 

 tube. Mercury is poured into the reservoir until the tube is filled to 

 the mark on the neck. The upper cock is closed and the mercury drawn 

 out into a tared vessel till the difference of level between the two mer- 

 curj^ surfaces in the manometer is a certain quantity li. After the mer- 

 cury is weighed it is poured back into the tube, the substance whose 

 specific gravity is to be determined is introduced into the reservoir and 

 the above operations are repeated. Simple calculation gives the value 

 required. — {Ann. Phys. Chetn., II, vi, 288, 1879.) Buignet has improved 

 the Nicholson areometer by making the stem longer and larger, marking 

 zero at the point to which it sinks in water when empty, and n at the 

 point to which it sinks when loaded with n grams. To use it, the in- 

 strument is placed in water and sinks to zero. The mineral whose grav- 

 ity is desired is placed on the upper pan ; the stem is immersed to a 



point P. Placed in the lower pan, it sinks the stem to a point, P'. Then 



P 

 pqp, is the density of the specimen. The sensitiveness of the instru- 

 ment depends on the size of the stem. Its use is rapid and accurate. — 

 (J. PJiys., ix, 93, March, 1880.) 



