276 Prof. J. S. Townsend on 
It is clear from the figure that perfection of design has, in 
the model, been sacrificed for the sake of simplicity of 
construction. Instead of having holes bored in FH the slot 
should be extended to F and another slot cut between G and H. 
A sliding focal pin could then be clamped in any position. 
Also the rhombus should have been shaped at the corners 
A and B so as always to leave sufficient space between the 
sides to allow of the pen P sliding right mp to the corners, 
even if the rhombus is nearly closed. 
To describe a parabola, F’ is made the focus, and FH is 
moved at right angles to itself, keeping it always parallel to 
its original direction. Under these circumstances P describes 
a parabola, and G its directrix. JI made no provision in my 
model for drawing parabolas, but I found that it would 
describe a very fair parabola if the flat end of F were made 
to slide along a fixed ruler. 
I have not previously published any description of these 
compasses, as I hoped some time to improve the design and 
get a good working instrument made. 

XXVIT. Phe Charges on Ions. By Joun 8S. TOWNSEND, 
F.RS., Wykeham Professor of Physics, Oxford”. 
HE relation between the charges on ions produced in 
gases by various methods is a matter of some im- 
portance, as the theory of electric currents in liquids and 
gases which is almost universally adopted is founded on the 
principle that all these small subdivisions of electricity with 
which the ions are charged are equal to or exact multiples 
of some charge which is absolutely fixed. The theory is 
supported by the phenomena which accompany the passage 
of electricity through liquids, and as is well known the 
charges on the ions are all exact multiples of the charge on 
the hydrogen ion in a liquid electrolyte. The theory also 
holds for gases ; and it can be proved that the charge on an 
ion produced by almost any of the known methods, in a gas, 
is identical with the charge on the hydrogen ion in a liquid 
electrolyte. 
It is of interest to collect the results upon which this 
theory is founded, and to show to what degree of accuracy 
the atomic charge may be considered to be known. 
If E is the charge on a hydrogen ion or atom in a liquid 
electrolyte, N the number of molecules per cubic centimetre 

* Communicated by the Author. 
