262 Mr. L. F. Richardson on a Freehand Graphic way 
to make the drawing, that the two surfaces g> — const., which 
pass respectively through the electric current and half-way 
between two adjacent turns of the current, must be surfaces 
of flow. Again, very close to the electric current the flow of 
heat will be nearly the same as that due to a straight current 
tangential to the helix, that is to say, the lines of flow will 
be normals to the helix and the isothermals will approximate 
to circular cylinders concentric about the tangent. 
r 
The particular dimensions chosen were -,=0*05 for the 
core, 0*3 for the source, and 0*5 for the outer cylinder. 
0-5 
Fig. 6. 
! f 
L 
-jP = 0-3 
, 
k 
005 
! 
i 
i 
. 1 _ > 
* 41 
AXIS 
OF THE SCR 
:ws 
Owing to orientation of the chequer affecting its shape 
this graph took twice or thrice as long to adjust as did the 
others in this paper. Its errors are discussed in Section VIII. 
hereafter. 
The magnetic field due to the helical current may doubt- 
less be determined in a very similar manner. Here ^ — will 
be a constant other than zero and the cyclical properties of 
the field will add a further complication. 
