o 
ME. LATIMER CLARK ON A STANDARD VOLTAIC BATTERY. 
The measure of quantity is the same as that of electrostatic capacity, and in practice 
generally receives the same name, although it has been sometimes called the “ Weber;” 
the weber or farad quantity is equal to 10 -2 absolute units. Electrical currents are 
defined as currents of so many farads per second. In this system the volt electromotive 
force through the ohm resistance produces the unit current, or a current of one farad 
per second. 
The Committee determined with great care the value of the ohm resistance and the 
farad capacity, and issued standards which have been very extensively copied and dis- 
tributed. They would naturally have desired to issue a standard of electromotive force, 
or degree of potential, and thus complete the series ; but in this they met with insuperable 
difficulties, and finally separated without accomplishing this part of their task. 
This was a matter of regret, seeing that the electromotive forces of batteries and the 
strength of currents are among the measures most frequently required by the practical 
electrician. 
The difference of potential between two bodies may be measured by measuring the 
force of attraction between two electrified planes of known dimensions at a known 
distance, or two coils conveying currents. It may also be determined by similar means 
to those employed by the Committee in their determination of the absolute unit of 
resistance — that is, by revolving a coil at a known speed in a field of known magnetic 
intensity. If the value of the earth’s horizontal magnetic intensity (H) were uniform 
at different times and places, or easily obtained, and if the measurements were made at 
a distance from iron bodies, the tangent galvanometer would afford a means of absolutely 
measuring electromotive force. 
All these methods, however, require complicated and expensive apparatus and great 
manipulative skill ; and owing to these causes it may be safely asserted that not more 
than half a dozen absolute determinations of potential have ever yet been made. Prac- 
tically electricians have been compelled to defi n e electromotive forces by comparison 
with those of the Grove’s or Daniell’s cell, the copper and zinc cell, or other electro- 
motive sources ; and it is a curious circumstance that among the thousand galvanic com- 
binations known to exist, not one has been hitherto found which could be relied upon 
to give a definite electromotive force : however pure the materials, and however skilful 
the manipulation, differences varying from four or five per cent, upwards constantly occur 
without any assignable cause ; and different observers using different materials of course 
meet with still larger discrepancies. 
The author, sustained by a conviction that this difficulty could not, in the nature of 
things, be insuperable, has carried on a course of experiments since 1867 with a view to 
discover and obviate the cause of these variations, and has devised a form of battery 
which he desires to lay before the Royal Society, and which appears to meet, in a very 
satisfactory manner, all necessary requirements. 
The battery is formed by employing pure mercury as the negative element, the 
mercury being covered by a paste made by boiling mercurous sulphate in a thoroughly 
