THE MAGNETIC PROPERTIES OF IRON. 
533 
14 inches long and 20 inches in circumference, and it can be run at a rate of one turn 
per minute or of six turns per minute, the speed being in each case almost perfectly 
uniform. The galvanometer lamp stand was provided with cross-wires, the galvano¬ 
meter with a light plane mirror, and the cross-wires were clearly focussed on the 
chronograph sheet by means of a lens placed between the lamp and the mirror. The 
chronograph sheets were, for convenience of experiment and subsequent reduction, 
formed of “ cross-section ” paper, the cross-lines on which were one tenth of an inch 
apart. 
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The method of experiment is simply to hold the finger on the switch ^ and observe 
the motion of the chronograph until the image of the cross-wires comes to a particular 
line on the paper sheet; at this instant close the switch, and then follow the image 
of the cross-wires with a soft sharp pencil, dotting the line occasionally, but taking 
care not to influence the motion of the chronograph. When the deflection has reached 
a maximum, the commutator is suddenly turned as the image of the wires passes a 
known position and the cycle of reversal marked out on the paper. The chronograph 
is now stopped, and the complete curve lightly marked. The experiment is then 
repeated, the image of the wires being simply observed as it passes over the curve, 
and any disagreement noted. Any disagreement being corrected and tested, the 
curve is ready for reduction. The time required to take a complete set of curves, 
check, and correct them, only requires a few minutes, after a little dexterity has been 
acquired in dotting out the curves. The curves obtained by this method have the 
advantage of being drawn to a large scale, the galvanometer sensibility being varied 
by means of shunts, so as to make the double deflection nearly cover the whole 
breadth of the chronograph sheet, while for many of the experiments made the length 
of the diagram extended several times round the drum. T am at present constructing 
an automatic recording apparatus on the principle of the Thomson recorder for the 
purpose of drawing diagrams for smaller masses of iron, where the interval of time is 
so short as to make the initial motion of the spot of light too rapid to be followed 
with certainty. Should this apparatus prove successful, I hope to apply it to invest!- 
