STUDIES ON MAGNETIC DISTRIBUTION 99 



The coils of wire which I used consisted of from twenty to one 

 hundred turns of fine wire wound on thin paper tubes which just fitted 

 the bar and extended considerably beyond the coils. The coils were 

 mostly from -1 to -25 of an inch wide and from -1 to -2 inch thick. A 

 measure being laid by the side of the given bar under experiment, the 

 coil was moved from one division of the rule to the next very quickly, 

 and the deflection produced on an ordinary astatic galvanometer noted. 

 After experience this could be done with great accuracy. It might be 

 better in some cases to have the coil slide over a limited distance on 

 the tube, though for the use to which I intend to put the results the 

 other is best. 



Up to 35 Q f is nearly proportional to the deflection; and when any 

 larger value is put down in the Tables, it is the sum of two or more 

 deflections. I have not the data in most cases to reduce my results 

 to absolute measure, but took pains to ensure that certain series of ex- 

 periments should be comparable among themselves. 



Having measured Q e at all points of a rod, we may find Q by adding 

 up the values of Q f from the end of the rod. 



The magnetizing force to which the bar was subjected was in all 

 cases a helix placed at some part of the bar. The iron bars were of 

 course demagnetized thoroughly before use by placing them in the 

 proper position with reference to the magnetic meridian and striking 

 them. 



In the Tables L is the distance in inches from the zero-point, Q f is 

 the deflection of the galvanometer when the helix is passed between the 

 points indicated in the first column. Thus, in Table II, 34-7 is the 

 deflection on the galvanometer when the helix was moved from the 

 tenth to the eleventh inch from the zero-point; and so we may con- 

 sider it as the value of Q f at 10 inches; so that the values of Q ( refer 

 to the half inches, but Q to the even inches. 



In all the calculations the constants in the formulae were taken to 

 represent Q most nearly, and then the corresponding formulae for Q e 

 taken with the same constants. 



For ease in calculating by ordinary logarithmic Tables, we may put 



-rL 1 /ymSrt 



IV. 



Table I is from a bar 17 inches long with a magnetizing helix 1 

 inch long at one end, the zero-point being at the other. Table II is 

 from a bar 9 feet long with a helix 4$ inches long quite near one end, 

 the zero-point being at 1 inch from the helix toward the long end. 



