of determining Stream Lines and Equipotentials. 261 
Of the innumerable solutions of V 2 V = possessing screw 
symmetry of: the sort described, which may be obtained by 
the aid of this standard chart, perhaps the simplest is the 
field due to a helical line source, such for example as the 
distribution of temperature in a mass of electrically insulating 
material which encloses a helical copper wire carrying an 
electric current. To avoid the introduction of a difficulty 
not characteristic of screw symmetry. I have assumed a core 
of non-conducting material in the form of a circular cylinder 
surrounding the axis. This relieves us of the necessity of 
considering the axial line of equilibrium, which would other- 
wise have to be treated by an extension of the method in 
Section IV. The external surface of the medium is also 
taken as a circular cylinder and is assumed to be at constant 
temperature. Consistently with our boundary conditions 
we may suppose that y-=0. 
Now symmetry will help us 
in several ways, for since the chequer ratio on the standard 
chart is the same whether any particular half-turn of the 
screw passes over or under the chart, one sees on beginning 
