Magnetic Fields on Metallic Resistance. 903 



It was impossible to get purely longitudinal fields in the 

 electromagnet. With the pole-pieces necessarily so far apart 

 it was inevitable that the field should be non-uniform. In 

 addition, the method of arranging specimens introduced a 

 small transverse component where the wires bent in a zigzag- 

 fashion were doubled back upon themselves. This effect was 

 reduced to a minimum by using fine wire and few turns. Its 

 magnitude could be very closely estimated by comparing with 

 the results obtained in the smaller uniform fields of the 

 solenoid for the same metal mounted more perfectly. 



In taking readings, the resistance was first measured 

 without the magnetic field, again when the field acted on the 

 metal, and finally another reading was taken without the 

 field. In all cases the magnetizing current was left on for 

 as short a time as possible so as to prevent heating effects. 

 The magnetic field of the electromagnet was measured with 

 a bismuth spiral. 



The data secured can best be examined when rfR/R, the 

 change of resistance divided by the resistance in zero field, 

 is plotted against H, the magnetic force. 



Plate XVI. 



Fig. 1. Iron. 



I. Longitudinal field, using solenoid. Temp. 23°. 



Soft Norway iron wire, diam. 0*032 cm. Resistance 

 2-89 ohms. 

 II. Longitudinal field, using solenoid. Temp. 23 0, 5. 



Piano wire, diam. 0*025 cm. Resistance 3*86 ohms. 



Fig. 2. Iron. 

 III. Longitudinal field, using electromagnet. Temp. 23°. 



Soft Norway iron wire, diam. 0*017 cm. Resistance 

 3*53 ohms. 

 IV. Angle of 45° between current and field, using electro- 

 magnet. Same specimen as in III. 



Fig. 3. Iron. 



V. Transverse field, using electromagnet. Same specimen 

 as III. 



Fig. 4. Nickel. 



I. Longitudinal field, using solenoid. Temp. 23°. 

 Diam. 0*050 cm. Resistance 1*25 ohms. 



Fig. 5. Nickel. 



II. Longitudinal field, electromagnet. Temp. 23°*5. 



Diam. 0*017 cm. Resistance 2*75 ohms. 

 III. Angle of 45° between current and field, using electro- 

 magnet. Same specimen as II. 



