Brown — Subsidence of Torsional Oscillations of Nickel Wires. 101 



clutch, to the lower side of the vibrator in the axis of rotation, and after 

 passing over two frictionless pulleys the other end had a scale pan attached 

 on which weights could be placed. — In this way a longitudinal pull was pro- 

 duced (in addition to that due to the vibrator) along the axis of the nickel 

 wire under test, and the string was carefully arranged so that there was no 

 torsion. Though this extra pull does not measurably change the period of 

 oscillation of the vibrator, it changes slightly the damping of the torsional 

 oscillations as shown below in Table I in the column marked (d.c). 



In order, therefore (with the present arrangement of one copper wire 

 down the middle of the iron tube), to be able to take the observations when 

 magnetic fields up to 200 c.g.s. units were on the nickel wire, it was arranged 

 to use throughout (1) a vibrating load on the end of the wire equivalent to 

 2 x 10 5 grammes per sq. cm. ; and (2) a load of 1670 grammes on the scale- 

 pan end of the silk string, making in all a longitudinal pull on the wire 

 under test equivalent to 2 - 3 x 10 5 grammes per sq. cm. 



The wire first tested was a No. 16 soft nickel wire of simple rigidity about 

 708 x 10 6 grammes per sq. cm. 



Observations on the subsidence of torsional oscillations were taken for 

 twelve different values of transverse magnetic fields up to 200 c.g.s. units, both 

 continuous and alternating, of frequency 50 per second, that is twenty-four 

 sets of observations in all. When the values are plotted with the number of 

 vibrations as abscissae and the corresponding values of the amplitude of 

 oscillation as ordinates, we get the curve of subsidence of torsional oscillations 

 referred to here as the damping curve. 



It was found that the application of a continuous transverse magnetic 

 field had no effect whatever on the damping of torsional oscillations, that is, 

 the damping curves were identical for no field and for a continuous field of 

 200 c.g.s. units; but that a transverse alternating magnetic field had an 

 effect on the damping curve, that is, for a magnetic field of the 200 units the 

 amplitude of the 70th vibration was decreased about 10£ per cent, from its 

 value with no field. 



In Table I are given the values for no field, or the earth's magnetic field only, 

 and for an alternating transverse magnetic field of 200 units ; the values for_ 

 the lower alternating transverse fields are not given in the table, but they all 

 lie between the values given in the columns marked D.C. and A.C. The 

 column marked (D.C.) gives the values of the amplitude of oscillation when 

 the weight of 1670 grammes was on the scale -pan end of the torsionless silk 

 string, that is, when the extra longtudinal pull through the string was on the 

 wire, and the column marked (d.c.) gives the values when the silk string was 

 off, there being no transverse magnetic field or a direct transverse field of 



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