232 PROCEEDINGS OF THE AMERICAN ACADEMY. 



S = 1.489 

 I) = 1.905 cms. 

 n = 50 turns 



B 100 • R 



we get ^ - 2(1.489) tt (0.9525)- • 50 ' 



The right-hand member is a constant for any given R In the work on 

 the series of curves the R had values ranging from 117 to 7117 ohms ; 

 the galvanometer and secondary coil circuit having itself 117 ohms, of 

 which the galvanometer had about 99 ohms, and the coil 18 ohms, 

 the other resistance being added, when convenient, from the resistance 

 box E '. The constants for these various ^'s were found and written 

 down. Then all we have to do to find the B for any observation is to 

 multiply the observed throw in centimeters by the proper constant. 

 This was done either by means of logarithms or a very good slide rule. 

 If we use the step-by-step method, the formula simply drops the 

 factor 2 and becomes, 



A5 100 R 



T S7r(D/'2yn 

 For the long solenoid we have simply 



H' = — V (No. of amperes used) 

 = 27.064 (No. of amperes). 



Having found the values of B and H', they were multiplied by 3 

 and 2 respectively, in order to facilitate the plotting of the points of 

 observation. Then the magnetization curves were drawn by free-hand 

 so as to fit the points as closely as possible. 



This gives us the curves from m= 15 to 200 in Figure 15. To find 

 the corresponding normal curve (m = cc) a graphical device was found 

 to be of the very greatest utility. Not only was an enormous amount 

 of time saved, which otherwise it would have been necessary to spend 

 in almost endless computations, but the device was a positive aid in 

 determining the position of the normal curve. On a large sheet of 

 tracing cloth were drawn about seventeen horizontal lines, so that when 

 properly placed over the sheet of millimeter paper on which the mag- 

 netization curves had been drawn, they coincided with the lines B = 0, 

 1000, 2000, etc., up to 16,000. By means of lines radiating out from 

 a point on the lowest of theee horizontal lines, each one of the lines 



