of High Specific Resistances. 465 



galvanometer to a megohm. The accuracy of this method of 

 testing has been already established. 



The distance from the galvanometer-mirror to the scale was 

 155 centimetres ; and the light-spot was very good, showing 

 the wire image as sharply as the lines on the scale. The 

 scale itself was divided into millimetres. In the final test the 

 period of vibration of the magnet system was 80 seconds, and 

 the resistance out of the Bridge-box was 100 ohms. The 

 E.M.F. acting through the megohm and galvanometer and 



100 ohms was therefore 1()1()() ('lark cells, say '0145 volt. 



•0145 

 The current was therefore ir =1/26 x 10~ 7 amperes. 



Employing the method of vibrations and neglecting the 

 correction for the extremely small log. decrement, the battery 

 being of course reversed and several experiments made, it was 

 found that the double deflexion amounted to 5 scale-divisions 

 (millims.). Hence the deflexion corresponding to 1*26 x 10 -7 

 amperes is 2*5 divisions. Now I do not think that, bearing in 

 mind the lengthy period of the sj-stem, it would be possible to 

 read to more than *5 division. The difficulty comes in in 

 eliminating air-currents ; as far as the scale went I could read 

 certainly # 2 division, so there is no advantage to be gained in 

 having the scale further, or even so far away. We may 

 therefore say that five times the least measurable deflexion is 

 given by 1*26 x 10~ 7 amperes, or that the least measurable 

 deflexion itself is given by 2*5 x 10 ~ 8 amperes. This, I may 

 mention, is only to be obtained when the magnets are 

 judiciously kept from swinging by an extra controlling 

 magnet, worked carefully from a distance till the vibrations 

 get small, so as to jam against the side of the coil. Some 

 fine quartz threads were also prepared by Boys' method, but 

 no appreciable improvement introduced by their use could be 

 detected. This may, however, have been on account of their 

 thickness ; for they could be seen, with a little practice, in a 

 good light, and when laid on a bit of black silk. I therefore 

 came to the conclusion that neither I nor my instrument- 

 maker (who is fairly good) could hope to compete in such 

 delicate work with the Messrs. Gray, who, using this type of 

 galvanometer, attained a sensitiveness of one half millimetre- 

 division with a current of about 10~ n amperes, and that with 

 a scale fairly close to the instrument, and with a manageable 

 period of vibration. 



In consequence of this failure I determined to use the four 

 coils constructed for the Gray galvanometer, for an expe- 



