the Magnetic Permeability of Nickel and Iron. 185 
the two secondary circuits AB //', CD gq’ have been pre- 
viously adjusted as nearly as possible to equality *. 
Fig. 3. 
z 
ttt 
S 
8 
S 
S 
ohana) {4 k 
SOLENO/D C/RCUIT 
LS 
S. Solenoid. C. Compensating-coil. P. Pt. tube. Q. Manganin 
resistance. M. Magnetometer-coil. A. Ammeter. 7. Rheostat. 
The resistance of the portion CD of the manganin 
‘“< standard ”’ was accurately determined beforehand. 
The platinum temperature, “pt,” is derived from the 
platinum resistance by means of the equation 
e— Ro 
j=] = 
a s . . e . ° I 
Roe ; ( ) 
where R and Ry are platinum resistances at ¢° and 0° 
* The two circuits never differed by more than 4 ohm, and since 
the resistance of the total circuit was 100 ohms, observations of platinum 
resistance lay within the limits of accuracy of the experiment. 
