APPENDIX 



167 



place a little before the galvanometer is un-short-circuited. The 

 removal of the two curved wires from their respective mercury 

 pools is maintained at a regular interval by the use of a weight. 

 This key is used to demonstrate the total effect between two points, 

 A B, after excitation of these same points. 



The second key is a little more complicated, and serves to 

 demonstrate a partial effect A C or B C after excitation of two 

 points A B. 



Units of Resistance and of Conductance* The units of resistance 

 are the ohm and the megohm (=1,000,000 ohms). The corres- 

 ponding units of conductance are the mho and the gemmho, which 

 are the reciprocals of the ohm and megohm. 



A substance having a resistance of one megohm (0) has a con- 

 ductance of I gemmho or i 7. 



The conductances of, e.g., the skin in the experiment illustrated 

 by Fig. 50 are accordingly : 



* I am indebted to Professor Ayrton for the suggestion of the reciprocal megohm 

 as a convenient unit of conductance, and for the further suggestion of " gemmho " or 

 7 as its convenient name and symbol, thus utilising Lord Kelvin's proposal to take 

 resistance names written backwards to denote their conductance reciprocals. Kohl- 

 rausch's unit of conductivity i K is the same as Kelvin's " mho," and is equal to 

 1,000,000 7. I have also to acknowledge the kind assistance of Dr T. .Martin 

 Lovvry in connection with conductivity measurements. 



