458 Prof. R. Threlfall on the Measurement 



Finally, the steady pins are taken out of the plates and the 

 gum is ready to be measured. 



In order to measure the resistance of good insulation by 

 means of this arrangement, it is clear that it will be ad- 

 vantageous to have a galvanometer of the highest degree of 

 sensitiveness. This is desirable both because the thicker the 

 insulating layer the less will be the experimental error in the 

 determination of its thickness ; and the smaller the electro- 

 motive force required the less will be the difficulty of estimat- 

 ing it exactly, as will be shown in the proper place. I there- 

 fore attempted to obtain sensitiveness by pushing the ordinary 

 conditions further than is usually done. My first experi- 

 ments were on a galvanometer of about 9000 ohms' resistance, 

 made by the Cambridge Scientific Instrument Company. It 

 was soon very evident that when the current reached the value 

 of about 10" 7 ampere, the torsion of the suspension became 

 important. My first modification was to increase the length 

 of the fibre to about 12 inches ; this led to considerable 

 difficulty of adjustment, but increased the sensitiveness about 

 fifty-fold. It then became clear that the next step must be 

 to get the magnets more perfectly astatic and to reduce the 

 weight of the mirror. The reducing of the weight of the 

 mirror turned out to be more difficult than I anticipated ; 

 however, it was finally arrived at, and at the same time the 

 astaticism was made more perfect. fSome experiments showed 

 that it was very difficult to get two sets of steel bars of the 

 kind ordinarily employed even reasonably astatic. The 

 difficulty lies partly in the magnetizing and partly in obtain- 

 ing exactly equal quantities of steel in the two systems of 

 magnets. In fact it is necessary that the steel bars be mag- 

 netized in situ, otherwise they can hardly be perfectly 

 arranged and are sure to demagnetize each other more or 

 less. Now when the two magnet systems are only separated 

 by a bit of aluminium, say three inches long, it is impossible 

 to thoroughly magnetize one system without demagnetizing 

 the other more or less. Consequently it is necessary to set 

 up an arrangement so that both systems can be magnetized 

 at once. The following is a description of the arrangement 

 adopted : — It consists of two small electromagnets with 

 extremely soft cores, and movable pole-pieces most carefully 

 worked so as to fit the ends of the cores. Every precaution 

 was taken to make the electromagnets as much alike as 

 possible; the iron was cut off the same rod, it was bent to 

 the same templet, the annealing of both cores was done in 

 a box of asbestos at the same time. The four brass bobbins 



