074: Mr. M. Siegbahn on the Study of Variable 



of a soft iron ring with four upright iron rods. Each of 

 these is covered with a solenoid of copper wire. The solenoids 

 have been arranged so as to close to 80 volts if coupled in 

 series. In the brass box are fixed four pole-pieces of the 

 shape indicated by fig. 4. Each of these is provided with a 



Fiir. 4. 



' c 



Dv 







D 



.-6.0 



cylindric plate which fits one of the four electromagnets. In 

 the magnetic fields NS, N'S' are extended two thin silver 

 wires (diameter 0'02 mm.), AB and CD. The tension in 

 these wires can be regulated by means of special screws. The 

 larger screws will be used for turning on the electric current. 

 At the crossing-point the two wires are insulated with a plate 

 of mica, and also provided with a small mirror. 



If a constant current is made to pass through one wire, 

 /. t. AB, one-half will be lifted, owing to the magnetic field, 

 the other will be pressed down ; hence the mirror will be 

 turned at a certain small angle. With sufficiently small 

 angles the turning is proportional to the strength of current. 

 Owing to the great tension in the wire, the new equilibrium 

 is not restored until after a considerable time of oscillation. 

 If the box is filled with a suitable damping fluid, this move- 

 ment can be made aperiodical. With the ordinary two- 

 stringed oscillographs, vaseline-oil is used for this purpose. 

 With this apparatus the vaseline-oil did not act as a sufficiently 

 strong damper. Caster - oil, however, proved effective. 



