12 REPORT ON THE COMPLETION OF 



The points of principal consequence are, well made hardened knife edges, and a 

 magnet that will not lose magnetism. The temperature coefficients of well hard- 

 ened thin bars, especiallj^ of those supplied by Robinson, have been very small ; 

 and it would not be difficult for the maker, by a few experiments, to diminish or 

 destroy the effect of temperature altogether in the manner suggested afterwards. 

 The only subject then remaining for the maker's skiU would be the formation of a 

 powerful and permanent magnet with a hard well-made knife-edge. I have 

 already shown in other places, than no dependence can be placed on the coeffi- 

 cients involving the time of vibration of this needle. 



Mechanical Temperature Compensation. — The difficulties connected with the 

 diminution of the temperature coefficients of magnets, for the purposes of the 

 bifilar and balance magnetometers, may be, to a great extent, avoided by mecha- 

 nical compensations. Such compensations, by a little experimentation and previous 

 calculation, may be sufficiently complete for most self-registering apparatuses, to 

 render considerations of the varying temperature negligible : even for the more de- 

 licate apparatuses, however, any incompleteness of the temperature compensation 

 may be determined by the process which I have employed. The following me- 

 thod of compensation may be adopted for the bifilar magnet : — Let the upper ex- 

 tremities of the suspension wires be attached to the ends of two brass rods, which 

 approach each other within an interval equal to the diameter of the lower wheel, 

 and let the other ends of the brass rods be fixed to a beam of wood, so that an 

 increase of temperature will cause the free ends of the rods to approach each 

 other, by an amount equal to the difference of their expansion and that of the 

 wooden beam to which they are fixed. Such an approximation will diminish the 

 directive force of the wires, and by a proper regulation will compensate for the 

 diminution of the magnet's moment. If the suspended wires are silver, and the 

 lower wheel is of brass, the coefficient {e) of contraction of the space betwixt the 

 suspension wires at the top will be found from the equation of equilibrium to be 



e = q 



where q is the temperature coefficient. Thus, in the case of the IMakerstoun 

 bifilar ^ = 0*000266, the interval of the wires is nearly 0-5 inch, and, therefore, the 

 brass rods would require each to be about 7^ inches long, in order that the in- 

 terval be diminished 0-000266 of itself, or 0-000133 inch; the difference of the co- 

 efficients of expansion of brass and wood being assumed =0-0000085. Mag- 

 nets with a temperature co-efficient of 00001 would require brass suspension 

 rods of 3 inches in length, or less in proportion as the interval of the wires is less 

 than 0-5 inch. 



For the balance magnet ; let a brass rod be fixed to the magnet near its south 

 end, but free to expand towards the north, and having as much to the north of 

 the axle as to the south ; it is obvious that when the temperature increases the 



