538 



PHYSICS, PROGRESS OF, IN 1901. 



lines of a four-pole induction motor, with an alu- 

 minum drum as rotor. The current passes 

 through one pair of field coils, while the other 

 pair are short-circuited on themselves. Horn con- 

 structs an electromagnetic voltmeter with two 

 scales, one in black for continuous currents, the 

 other in red for alternate currents. Two short 

 cylinders of sheet iron are supported eccentrically 

 in a solenoid, one on each side of the axis and at 

 each end of the solenoid. Their motion, under 

 the influence of the field in the solenoid, tends to 

 place them coaxial with the solenoid, and it is 

 transmitted to the needle by a jointed frame. 

 A novel exhibit is Arno's electrostatic voltmeter, 

 based on the principle that a dielectric in a rotary 

 electrostatic field tends to rotate with the field 

 by reason of the lag in its polarization. Three 

 segments of a metal cylinder, connected to a 

 three-phase supply, surround a paraffined paper 

 cylinder whose rotation is controlled by a spiral 

 spring. 



Resistances. Constantan is employed in the 

 construction of some recent resistances, and to 

 avoid the effect of this alloy's high thermoelectric 

 power the terminals, slides, and all fittings are 

 also of constantan. In high resistances made by 

 Kundt's process, a spiral line is painted on a 

 porcelain tube with a mixture of gold and plati- 

 num chloride in camomile oil. This spiral is 

 reduced to the metallic state by burning in a 

 muffle, and then possesses a very high resistance. 



Potentiometer. A portable potentiometer was 

 shown, having a slide- wire contact adjustable rel- 

 atively to its index, so as to compensate for the 

 temperature variation of the standard cell. 



Ohmmeters. Several direct-reading instru- 

 ments with one fixed and one moving coil were 

 shown. Another ohmmeter had two fixed coils 

 acting on a soft-iron needle, to which was at- 

 tached a pointer prevented from oscillating by a 

 fork. The fork can be moved by a milled head 

 to the position of equilibrium. 



Earth Detectors. -In an electrostatic voltmeter 

 for indicating the insulation of live conductors, 

 the two conductors of a line are connected to the 

 two pairs of quadrants, while the other needle is 

 connected to earth. If the insulation of one con- 

 ductor diminishes, the needle deflects toward the 

 other quadrants. 



Condensers. Standard condensers are now 

 sometimes made of sheets of mica silvered by 

 chemical deposition. A condenser having a very 

 small residual charge is made of chemically pure 

 paper impregnated with an insulating material, 

 the nature of which is kept secret. 



Wattmeter's. With few exceptions, those exhib- 

 ited were of the electrodynamometer type. Sie- 

 mens and Halske make non-inductive resistances 

 for connecting in series with the pressure coil of 

 their wattmeters by winding flat wires on thin 

 sheets of mica, fixed side by side in a box about 

 7 millimeters apart. 



PJiascmeters. In the Siemens type two coils, 

 at right angles, carry the two currents, and pro- 

 duce a rotary field whose amplitude varies with 

 the difference of phase between the currents. This 

 amplitude is measured by the rotation of a metal- 

 lic disk, embraced by the coils, and controlled by 

 a spring. In Arno's phasemeter an electrodyna- 

 mometer with torsion head has two coils that 

 carry the two currents, and inside them is sus- 

 pended a pair of coils perpendicular to each other 

 and short-circuited on themselves. This system 

 is also provided with a torsion head. From the 

 readings of the two torsion heads may be calcu- 

 lated the tangent of the angle of phase differ- 

 ence. 



Curve Tracer for Alternate Currents. Abra- 

 ham's rheograph has a galvanometer coil of very 

 long period. In order that the force acting on the 

 coil may be at all times proportional to the cur- 

 rent this is transformed by two pairs of flat coils, 

 one within the other, and relatively adjustable 

 in plane. The fourth coil is in series with' the 

 galvanometer coil and with an adjustable resist- 

 ance in the main circuit of the first coil, and also 

 receives an induced electromotive force impressed 

 by the first coil, and adjusted by varying the rel- 

 ative inclinations of the planes of the coils. These 

 adjustments are made till the galvanometer cor- 

 rectly registers the curve of an interrupted con- 

 tinuous current. 



Permeameter s. Carpentier's permeameter con- 

 sists of a massive iron ring with two air-gaps 

 diametrically opposite. In one is suspended a 

 magnetized needle with torsion head and scale. 

 The test-rod, surrounded by a magnetizing coil, 

 is fixed across the ring on a diameter perpendicu- 

 lar to that on which are the air-gaps; the induc- 

 tion in the test-rod 'is then proportional to the 

 torsion required to bring the needle to zero. The 

 effects of hysteresis are eliminated by a double 

 reading. A complete apparatus for measuring 

 permeability by means of a bismuth spiral con- 

 tains an iron yoke to take the test-pieces, an am- 

 meter, and a slide-wire resistance bridge to meas- 

 ure the variations in resistance of the spiral. A 

 permeameter designed by Kath consists of a d'Ar- 

 sonval galvanometer, an iron yoke with mag- 

 netizing coil replacing the usual permanent mag- 

 net. The test-rod completes the magnetic circuit 

 of the yoke and carries the coil. A constant cur- 

 rent is passed through the moving coil, the index 

 of which moves over an arbitrary scale. 



Magnetism. Magnetization. I. Klemencic 

 (Sitzungberichte, of the Vienna Academy of 

 Sciences, 109 Ila, p. 827) discusses the decrease 

 of magnetic moment with time, and assigns the 

 loss chiefly to two causes. The after-effects of 

 magnetization are responsible for the greatest loss, 

 occurring in the first two days after magnetiza- 

 tion. The other cause, which acts more slowly, is 

 the change in molecular structure brought about 

 by previous deformations, and especially by former 

 temperature changes such as those by which the 

 steel is hardened. The greatest loss of moment 

 takes place in summer, little or none occurring in 

 winter. K. Kriise (Sitzungberichte of the Vienna 

 Academy, January-March, 1900) has subjected 

 bar magnets to violent treatment, and meas- 

 ured the resulting change of magnetic moment. 

 It is found that the greatest decrease is after the 

 first fall, and that the moment seldom remains 

 unchanged after frequent falls from the same 

 height, the total decrease varying between 1.6 

 and 25.6 per cent. When the magnets are pulled 

 from contact with a soft iron bar the moment 

 increases slightly after the first separation, chang- 

 ing to a decrease after ten separations. Drawing 

 a magnet from a soft iron plate sometimes dimin- 

 ishes its moment 28 per cent. 



Residual Magnetism. P. Holitscher (Annalen 

 der Physik, December, 1900). in experiments on 

 an ellipsoid of Jura iron, by magnetometric meas- 

 urement, finds that the residual magnetic mo- 

 ment reaches a maximum only after magnetizing 

 for a certain time. Repeated magnetizations, if 

 in one direction only, give continually increasing 

 residual moments up to 100 magnetizations, be- 

 yond which the author did not go. If the succes- 

 sive magnetizations are alternately in each direc- 

 tion, the maximum residual moment is acquired 

 after the first or second, and is about 12 per cent, 

 lower than that when the iron is magnetized in 



