94 Barrett, Brown & Hapristp—On the Electrical Conductivity and 
Comparing 1178 B with 1167 D in Group 5, which has the same amount of 
aluminium, it will be seen that the addition of 1°75 per cent. of chromium 
reduces the conductivity only 20 per cent. The greater effect produced by 
aluminium than by chromium in lowering conductivity is seen by comparing 1179 B 
and 1178 D. The specimens marked 1178 are similar chromium steels to which 
varying amounts of aluminium have been added. ‘The increase of resistance 
thereby produced is very marked, between 1178B and 1178 E the resistance 
increases at the rate of 11 microhms for 1 per cent. of aluminium added to 
these chromium steels, practically the same increase produced by the aluminium 
as when the chromium was absent (see p. 83). This rate is taken between 
0°75 and 4°5 per cent. of added aluminium ; between 0 and 1 per cent. the rate of 
increase of resistance is still higher, as might be expected. By comparing the 
resistances of the aluminium steels in Group 5, p. 83, with each other, and with a 
corresponding carbon steel without aluminium, the rate of increase of resistance for 
1 per cent. of aluminium added to iron will be found almost the same as the rate 
deduced from these composite steels. ‘The following table shows these results :— 
Average increase of specific resistance for each | 
Percentage of Aluminium in Alloy. i ra Gem, Git atkiloal allman 
| From 0 to 2:0 per cent. 12 microhms. 
nage aioe 11 3 
Serie aes 5 10 a 
yy IPB BB op 95 ,, 
The next group shows the effect of adding silicon to chromium steels. 
Group 18.—CuRomium-Sinicon STEELS. 
Marks. Percentage Composition. Conductivity Copper = 100. ee ee 
Cr | Si C Unann. Ann. Unann. Ann. 
518 2°0 1:00 0:76 4:70 54 36°4 31°8 
517 2:0 1°80 0:86 3°40 3°9 51:0 44:1 
1185 F 3°53 2°25 0°54 3°14 a4 54°8 50°6 
The enormous increase in resistance due to silicon is seen by comparing 
517 and 518; also by referring to 11771 in Group 7, a similar chromium steel 
to 1185 F, but without the silicon. This comparison shows that the rate of 
increase for each 1 per cent. of added silicon is 15 microhms between 1 and 1:8 
per cent. of silicon, and 11 microhms between 0 and 2°25 per cent. of silicon. 
