19 
Mr. Harris on the relative powers of, &c. 
considered as little more than an air thermometer, the metal to 
be examined being drawn into a wire and passed air tight 
through the bulb. There is a glass tube, whose interior 
diameter is regular, and somewhat less than -^th of an inch ; 
one of its extremities is bent upwards and outwards for about 
two inches, and is united by welding to a short glass cup ; 
this last contains a small quantity of coloured spirit. The 
opposite leg of the tube is sustained by a graduated scale, 
fixed upon a convenient base. The point at which the co- 
loured spirit rests in this leg is marked zero. Upon the cup 
is screwed a glass ball of 3 inches diameter, having the 
metallic wire to be examined passed air tight across its centre. 
To effect this, there are two flanches of brass carefully 
cemented about the holes in its sides, each flanch has a small 
projecting shoulder, which receives the wire, and upon which 
is finally screwed a small brass ball ; this ball has a flattened 
part to bear against a similar part of the flanch, and thus by 
a thin collar of leather, the whole is rendered air tight. The 
wire is secured firmly in its situation by means of a small 
peg of wood, and the holes in the balls are sufficiently deep 
to allow both the extremity of the wire and the peg to project 
a little, for the convenience of removal ;* thus the substitution 
of one wire for another is very simple and expeditious. 
Besides these flanches and balls, the bulb is also furnished 
with a sort of valve, attached in a similar way to its upper 
part, which being rendered air tight by a screw, can be occa- 
sionally opened so as to form a communication with the 
external air, and thus the coloured spirit may at all times be 
adjusted to zero. Fig. 2. Under these circumstances when 
an electrical explosion of sufficient force is passed through 
