On the Measurement of High Specific Resistances. 453 



unwilling to do so, because I have long been of the opinion 

 that the galvanometer as usually constructed is susceptible of 

 considerable improvement for delicate work by simply pushing 

 the ordinary conditions of sensitiveness nearer to their limit. 



General Description of Method. 



Measurement of Resistance of Gums. 



The substance to be tested forms a layer of very exactly 

 estimable dimensions between two plates of conducting 

 material. The experiment consists in obtaining equal de- 

 flexions of a sensitive galvanometer — (1) when a known 

 fraction of the E.M.F. of a Clark cell is allowed to act 

 through a megohm in the galvanometer circuit ; and (2) when 

 the E.M.F. of a known number of compared Clark cells is 

 allowed to act through the resistance to be measured. 



The apparatus, therefore, consists of the plates with the 

 substance to be investigated, the galvanometer, the standard 

 cells, and a megohm. 



The Resistance-plate Arrangement. 



This consists of two brass plates accurately rectangular and 

 scraped flat on one surface. The dimensions of the plates I used 

 were — length 15*2 centimetres, breadth 12'7 centim., thick- 

 ness 1'6 centim. These rather exact numbers were obtained 

 by filing. The measurements were made (a) by beam com- 

 passes, (yS) by the dividing-engine. Neither the corrections 

 which had to be introduced for temperature nor the com- 

 parison of the dividing-engine scale and the beam compasses 

 with the standard metre are given, as no absolute measure- 

 ments of pure substances have been made. Several measure- 

 ments of each plate were made by both methods. The surfaces 

 were made flat by scraping, and this process was continued 

 till the contact was sufficiently perfect for one plate to lift the 

 other when laid on it, both surfaces being clean in the 

 ordinary sense. The upper plate is furnished with a solid 

 handle something like the handle of a flat-iron, and is pierced 

 by three holes, through which pass the micrometer distance 

 screws (see Plate XIV.). The screws are 4 centim. long, and 

 the threaded portion is '55 centim. in diameter (they would 

 have been better if twice the diameter). The micrometer- 

 heads are divided into a hundred parts each, and the mean 

 pitch of the screw, as determined by a comparison with the 

 millimetres of a standard scale by means of a measuring 

 microscope, is 39*5 divisions to a millimetre ; that is, one turn 

 is equal to *5063 millimetre at 20° C. The points of the 



