A STANDARD OF ELECTRO-MOTIVE FORCE. 
785 
The only objection to this form is that the cells cannot, without precaution, be 
supported from underneath. Most of mine are held at the centre by a spring (cut 
from sheet metal) against a piece of board mounted on its edge. In this case the 
copper electrodes are secured in sealing-wax to the wooden stand. For single cells, 
when portability is desired and convenience of immersion in water or ice, it is a good 
plan to enclose the whole in a rather long and narrow test-tube. A little cotton wool 
supports the cell and prevents it from shaking about laterally. The gutta-percha 
covered leads pass through a piece of cork inserted near the top of the test-tube, and 
a little marine glue poured over the cork makes all tight. In order to give mechanical 
support to the platinum wire, which is liable to break where it passes through the 
glass, the external application of sealing-wax is recommended—a precaution applicable 
also to the H-cells. 
§ 43. In charging the cells the first step is to pour in sufficient pure" mercury to 
cover the platinum effectively. The paste (of which more presently) is next intro¬ 
duced, with the aid of a small funnel, care being taken not to soil the sides above 
the proper level. The zincs, cut from rods of pure zinc, as supplied by Hopkins and 
Williams, and not recast, are soldered to copper wires and cleaned in the lathe. 
Just before use they are dipped in dilute sulphuric acid, washed in distilled water, 
and dried with a clean cloth or filter paper. Each zinc is mounted in a shoit piece of 
cork fitting the tube (but not too tightly), and nicked in order to allow of the passage 
of air. The cork is pushed gradually down until its lower face is almost in contact 
* Except when the contrary is stated, mercury distilled in vacuo has been used for Clark cells. There 
is, I believe, a difficulty in purchasing mercury thus treated ; but every physical laboratory should be 
provided with an apparatus for this purpose. That employed by me was distilled at Cambiidge in an 
apparatus set up by Mr. W. N. Shaw. 
