HALL. — CONDUCTIVITY OF SOFT IRON. 141 



and is easily dealt with if we know the ratio between the thermal con- 

 ductivity, jfiT, of the platinum and its " surface emissivity," E, in the 

 stream of water. Assuming E to be the same for platinum as for copper, 

 and A" to be ^ as great for platinum as for copper, we get iov E -=r A the 

 value \. We now have (see Preston's " Heat," p. 513) 





or /x = 9 in round numbers. Then, using the formula ^ = ^q ^ ~ '^'^j where 

 6o is excess of temperature of the heated end of wire above temperature 

 of water, 6 the excess of temperature at any point distant x cm. from this 

 end, and c the Napierian base, we have, reckoning 6 in per cents of a 



Mean 

 2.4 % of 1° 



Accordingly the mean excess of temperature of the wire along its first 

 0.5 cm., less than oV of its whole length, would be about 5 % of the 

 usual difference of temperature, about 0°.5, between the two spirals; 

 and beyond this point the excess would be very small ; so that the error 

 made by neglecting the difference of temperature between the spirals and 

 the water is not important, provided the calculation just made is tolera- 

 bly accurate. The most uncertain element in this calculation is probably 

 the value of the " surface emissivity," which is based on observations 

 made on the behavior of the disk and the streams across its face. But 

 as the velocity of the water in passing the spirals is probably ten times 

 as great as its mean velocity across the disk, it seems altogether likely 

 that a sufficiently low estimate of emissivity has been used in the calcula- 

 tion, and that the possible error from the source in question has been 

 overestimated in the discussion just given. 



