TRANSACTIONS OF SECTION A. 653 



motion of an unresisted projectile is reduced to the drawing of geodetic lines on a 

 •certain figure of revolution of which the explicit equation, 



y=p^^/(.^•*-l), 

 is expressed in terms of elliptic functions. 



TUESDAY, AUGUST 9. 

 The following Reports and Papers were read : — 



1. Report of the Committee on Electrolysis. — See Reports, p. 72. 



2. Beport of the Committee on the Discharge of Electricity from Points. 



See Reports, p. 72. 



3. A Magnetic Curve Tracer. By Professor J. A. EwiNG, F.B.S. 



The instrument traces complete curves for cyclic or other processes of magnetisa- 

 tion. The material whose magnetisation curve is to be determined is arranged to form 

 a magnetic circuit with a narrow gap in it, and in this gap a conducting wire is 

 tightly stretched. A constant current passing along this wire causes it to sag up or 

 ■dovTU across the lines of force in the gap. Its movements, which are proportional 

 to the magnetism of the magnetic circuit, are communicated to a mirror which is 

 pivoted on a single needle-point, and is capable of being deflected in azimuth 

 as well as in altitude. The sag of the wire gives the mirror movement in 

 altitude. At the same time the mirror receives azimuthal deflection from another 

 stretched wire, which is stretched in a narrow gap between the poles of another 

 magnet. This second wire carries the variable current which is used to produce 

 the magnetisation of the first magnetic circuit, and the magnetic field in which it 

 is stretched is kept constant. Its movements are therefore proportional to the 

 magnetising current of the main circuit. In this way the mirror receives deflec- 

 tions in azimuth which are proportional to the magnetising force as well as deflec- 

 tions in altitude which are proportional to the magnetisation. Complete curves for 

 such operations as double reversal of magnetism are readily traced out, and may 

 be photographed by allowing the spot of light to fall on a sensitive plate, or may 

 be marked by following its movements on a sheet of paper with a pencil. The 

 instrument allows all the phenomena of magnetic hysteresis to be examined with 

 great readiness. The magnetic circuit which is under test is arranged in such a way 

 that the specimens of metal composing it may be quickly removed and replaced by 

 others. The instrument thus forms a practical magnetic tester capable of workshop 

 use in determining the magnetic qualities of any samples of iron or steel. When 

 the magnetic circuit is laminated the instrument allows magnetic reversals to be 

 performed with sufiicient speed to make the mirror trace out a continuous line of 

 light upon the screen. For very high speeds a pair of independent mirrors may 

 be preferred, arranged to give the beam of light its two components of motion by 

 successive reflexion. The arrangement is equivalent to the combination of an 

 extremely dead-beat galvanometer with an equally dead-beat magnetometer. The 

 galvanometer portion may be used alone in cases demanding the use of a current- 

 measuring instrument with a very high frequency of free oscillation ; and by 

 making the magnetic fields in both gaps constant, and sending variable currents 

 through both the stretched wires, the relations of the currents as to period and 

 phase may be studied. 



The instrument exhibited is sufficiently dead-beat to allow cycles of magnet- 



