998 Prof. J. A. Fleming on a Hot-Wire Ammeter for 
a vertical strip of ground glass 8, carried in a brass grooved 
frame which can be adjusted to any height on a vertical 
metal rod. The height of the incandescent lamp is so ad- 
justed that the lens forms a clear image of the filament or of 
one leg of the filament upon the ground glass in the form of 
a horizontal line of light. With a good lens this image can 
be made very sharp. The lens actually used was the objective 
of an old opera-glass. A hood of metal or asbestos placed 
over the lamp prevents the direct rays of the lamp falling on 
the ground-glass screen. The screen can be conveniently 
placed about a metre from the wire box. | 
If, then, a small current is passed through the slacker 
of the two measuring wires, its sag will increase and the 
small mirror attached to the two wires will be tilted, and 
the image of the filament on the ground glass will move 
down, but return again to its original zero, as soon as the 
current is removed. | 
As a preliminary step, both the wires must be aged by 
sending intermittently a small current through them for a 
considerable time, this current being continually interrupted. 
In the instrument actually made, the platinoid wires have 
a resistance of about 168 ohms each ; hence, if an electro- 
motive force of 2 volts is applied to the ends of the wires, a 
current of about 1/84 of an ampere passes through them. 
The instrument is calibrated in the following manner :— 
A secondary cell having a measured electromotive force, say, 
of about 2 volts, is connected in series with one of the working 
wires through a resistance-box of the usual plug pattern. 
By varying this resistance, different currents are passed 
through the wire and the position of the spot of light on the 
sereen corresponding to the different currents is noted. — 
If the wire employed is of platinoid or of constantan, its 
resistance will not be altered appreciably by different small 
currents passed through it, and hence the resistance of the 
wire can be determined once for all, with a sufficient degree 
of approximation for practical purposes, by means of a 
potentiometer. When this has once been done, a few obser- 
vations taken with a cell of known electromotive force and 
a plug resistance-box used as above, enable the observer to 
mark off on the ground-glass strip with a pencil the position 
of the line of light for various known currents lying within 
a certain range. The strip of ground glass may then be 
removed and applied to a sheet of squared paper, and a curve 
plotted down showing the deflexions in terms of the actual 
currents. This curve proves to be a parabola (see fig. 3) 
because, if we plot the logarithms of the deflexions and the 
