the Magnetic Permeability of Nickel and Iron. 181 
wire, wound non-inductively round them. He was the first 
to avoid oxidation by heating the specimens in a vacuum. 
Values of the critical temperatures obtained by these 
observers are :—for Nickel, about 320° ; 
for Cobalt, about 1100° ; 
for Iron, between 770° and 870°. 
General Method Employed. 
In the present investigation, the iron and nickel specimens 
are in the form of straight wires, 20 cms. long. A magneto- 
metric method is used for determining the induction, and the 
specimens are inclosed in an electrically heated platinum 
tube, the temperature of which is deduced from its resistance *. 
This method was suggested by Professor Callendar and 
described by Pitcher in the beginning of his paper (Phil. 
Mag. May 1899). 
Description of Apparatus. 
The magnetizing solenoid is 50 cms. long, and consists of 
two layers of double silk-covered copper wire (diam. 0°0356 
em.) wound on a brass tube (M, fig. 2) 2°68 cms. diam. 
The layers are carefully insulated from each other and from 
the brass tube by means of paraffined paper, and the windings 
are correct to 1 per cent. The lower layer contains 17° 35 
turns per cm., and the upper layer 17°54 turns per cm. 
By means of a second brass tube, concentric with the first, 
an annular space is formed between the two tubes, through 
which cold water can circulate. This water-jacket protects 
the coil from the effects of the hot platinum within, and also 
prevents undue heating by the magnetizing current. 
The magnetometer ‘consists of a ring-magnet hanging 
inside a cavity in a mass of copper. Vertically beneath 
the ring is a bar-magnet, the position of which can be 
adjusted so as to allow of variation in the controlling field. 
The instrument is also provided with a one-ohm coil mounted 
axially with the ring-magnet. The specimen with its 
surrounding solenoid is arranged horizontally East and West, 
with the magnetometer a few centimetres from the end of the 
solenoid tube (fig. 1). 
The coils C,C, are in series with the heating and magnetizing 
currents respectively, and consist of a few turns of thick 
* The nickel wires were 20 cms. long and 0°0765 cm. diameter 
(300 diameters in length). 
The iron wires were 20 cms. long and 0:0575 cm. diameter (400 
diameters in length). 
