of Large and Small Alternating Currents. 95 
The new thermal instruments now to be described were 
designed to get over one or more of the above difficulties for 
special purposes, and do not pretend to have overcome the 
whole of them. The first instrument was designed to be very 
quick in action for a thermal instrument, and was made to 
work with a small current so that it could be used asa quick- 
acting voltmeter. It is essentially a very delicate Ayrton- 
Perry twisted strip-ammeter which has been improved by the 
addition of a temperature-compensation device to minimize 
the zero-creep when the temperature of the whole instrument 
changes. The working parts of the instrument are shown in 
fig. 1. ABCD is the Ayrton-Perry twisted strip, of which 
the part A B is twisted in one direction and the part CD in 
the opposite. A mirror M anda very thin mica damping- 
vane are fixed at its centre. This strip is stretched in a frame 
formed of a brass block T, carrying one terminal and a piece 
of ebonite HK, the sides of the frame being formed of wires W W. 
This frame itself is stretched by means of a spiral spring § 
fixed to the other terminal block T,. i 
The twisted strip A BC D and the wires W W are made of 
the same metal ; in fact, the strip ABC D is actually flatted 
from the wire WW, so that the twisted strip and wipes 
have the same temperature-coetticient of expansion. If the 
wires and strip rise in temperature equally, then the whole 
frame HK WT, W simply gets longer and no twist. of the 
mirror ensues. If, however, a current be sept from T, to T, 
