
the Measurement of very small Alternating Currents. 597 
of this rod are fixed two small brass uprights to which terminal 
serews are attached and also small spring pieces of brass, p p, 
which are pressed in by screws passing through the uprights 
(see figs. 1 and 2). 
To meen springs at each end of the rod are attached fine 
wires, either of pure silver or of some high resistance alloy, 
such as constantan, ee &e., according to the use to 
which the instrument is to be placed. 
In the instrument I have already constructed, these wires 
are of platinoid, the length of the wires being one metre and 
the diameter 0:05 of a millimetre. The distance apart of 
these wires is about 5 millimetres. The extremities of these 
wires are soldered to the two spring pieces at the ends of the 
wooden rod, and the tension of these wires can be adjusted 
by means of the screws passing through the small uprights 
and pressing against the spring pieces. 
To the centre of the wooden rod carrying the above- 
mentioned fine wires are fastened two very delicate spiral 
springs, s, which have their other ends looped over the long 
straight wires. These spiral springs are made of extremely 
fine platinoid wire, and they serve to keep the ammeter wires 
tight (see fig. 2). 
Fig. 2. 
Lo) 



If one of the wires is heated by passing a current through 
it, it sags down slightly. This sag is indicated in the 
following manner :—The two wires are embraced by an 
exceedingly small loop of paper m made from a strip of paper a 
couple of millimetres in width and about 12 or 15 millimetres 
in length. 
To this loop of paper is attached with a touch of shellac a 
fragment of silvered microscopic glass about a couple of 
millimetres in width and 5 millimetres in length. 
The tension of one of the wires is so adjusted that when 
no current is passing through either of them one wire sags 
more than the other, and this little loop of paper and its 
attached mirror sets itself at an angle of about 45 degrees to the 
horizontal. This is attained by slightly relaxing the tension 
on one of the wires. Upon the lid of the containing box is 
carried an incandescent lamp, having a straight or horseshoe- 
shaped filament, and in front of the box is placed (see fig. 1) 
