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the electric resistance is an important factor of all iron 
and steel plates used in the construction of boilers, fire boxes, 
etc. It is important that the plates exposed to the fiame 
should conduct heat well, and one of the advantages claimed 
for steel over iron, is that as a thinner plate can be used, 
it allows more heat to pass than the thicker iron one. 
Thus a J-inch steel plate can replace a |-inch iron plate, 
but our experiments lead us to believe that as steel conducts 
electricity worse than iron the thin steel plate may, after 
all, let less heat through in a given time than a thick iron 
one. 
Suppose the iron plate has a resistance of *90 Ohms, per 
meter gramme, and the steel plate a resistance of 111 Ohms., 
then a f -inch iron plate will allow as much heat to pass per 
unit of area as a half-inch steel plate. If the steel plate be 
more highly carbonized, say of Siemen’s Martin steel, then 
the J-inch steel plate will allow no more heat to pass per 
unit of area than an iron plate -j^e'-ii^ch thicker. There is 
not space here to discuss this subject fully, but there is no 
doubt of its importance, and it is probable the time will 
soon come when our leading boiler makers will electrically 
test their steel plates, perhaps by the induction balance, just 
as they now test them for ductility and tensile strain. 
