Mr. J. N. Hearder on Electrical Conductivity. 15 
the best which afford the least. Upon this hypothesis, increased 
transverse sectional area in the same metal diminishes resistance 
by allowing the transmission of a larger quantity in a given time. 
The practical determination of the relative conducting capabili- 
ties of different metals, or of different samples of the same metal, 
has generally been accomplished by the comparison of galvano- 
metric or electro-magnetic effects; but I am not aware of any 
course of experiments which have been undertaken with a view 
to trace any connexion, or institute any comparison, between the 
thermal effects of the voltaic current on metals and their con- 
ducting powers as thus indicated, or to work out any scale of 
the conducting powers of metals, based simply upon the thermal 
effects of the voltaic current upon them, 
In 1826 Sir W. S. Harris communicated to the Royal Society 
the result of a series of experiments with his thermo-electrometer 
for determining the relative conducting power of metals for the 
Leyden discharge. His experiments were based upon the 
assumption that metals are heated by equal discharges of elec- 
tricity through them, from an electrical jar or battery, in pro- 
portion to the resistance which they offer to its passage ; hence 
their relative conducting powers in the scale were considered to 
be inversely as their thermometric indications. Thus in passing 
a carefully measured shock through wires of various metals, all 
of precisely the same diameter and length, stretched through 
the bulb of an air-thermometer, the relative degrees of heat in- 
dicated upon the scale are shown in the following Table, extracted 
from the Philosophical Transactions of 1827 :— 
Metals. Effects. 
CEU SERRA Ge Se NE Cate ge 
IVED ee oo erame 10 
Set eee see a) ea Td 
PING Oeste, ee asd el 
Platine nea OO 
LTV TTD Wag dette Reine ys hd ea ahs 4) 
HCl Sow era tei Sr ere Pi 
1b GEE hie laa laine oh lig bets ag? 
IBTGRS Ts Meo dah cases Bek 
In the year 1827 I thought of using this thermo-electrometer 
for determining the relative conducting powers of metals for 
voltaic electricity, and was pleased, on applying a single pair of 
plates to it for the first time, to find its indications extremely 
regular, the fluid rising constantly to the same point at each 
successive contact, and remaining almost permanent as long as 
contact was maintained. The instrument, however, appeared to 
require some few modifications to adapt it more particularly to 
