366 Crook — Yoke with intercepted Magnetic Circuit 



slot S may be varied in width or be completely closed up at 

 pleasure. Usually, however, the width is reduced to one or 

 two-tenths of a millimeter. It was found that even within 

 these narrow limits it was possible to use a test coil of suffi- 

 cient number of turns to give accurate readings for weak 

 fields. This coil C was constructed so that the total induction 

 in the yoke would pass through it. It consisted of 125 turns 

 of No. 46 double silk-covered wire between two thin sheets 

 of mica. That part of the coil which had to pass through the 

 slot was made of one layer of wire and the total thickness was 

 such as to pass with perfect freedom through the slot when 

 the latter was adjusted to a width not greater than -025 cm . Its 

 resistance was approximately 1000 ohms. 



The specimens of iron or steel to be tested are shaped into 

 cylindrical rods 40 cm long with a cross-section of '28 [] cm. 

 These fit into cylindrical grooves in the ends of the yoke and 

 are clamped in place by iron covers and brass set screws. The 

 magnetizing solenoid should be as small as possible. The 

 solenoid which was used was constructed for another purpose 

 and was much larger than necessary for bars of these dimen- 

 sions. This solenoid " M " was 19 cm long and consisted of 

 1000 turns of No. 16 double covered copper wire. It was 

 wound in nine layers on a brass spool, the inside diameter of 

 which was 2\5 cm . The rod when in position is coaxial with 

 this coil. 



The induction in the yoke is easily measured by jerking the 

 test-coil from the slot and reading the deflection produced on 

 the ballistic galvanometer. To determine the induction in 



