130 POPULAR LECTURES AND ADDRESSES. 



the required resistance is given by the tangent 

 of the angle through which the coils must be 

 turned to bring the needle to zero. A similar 

 instrument to give conductivity by a simple 

 reading, without any adjusting or " setting " for 

 each observation, is easily made. I made such 

 an instrument in 1858, being simply a galvano- 

 meter with controlling resistance coils instead of 

 controlling magnet. 1 Such an instrument at once 

 gives conductivity, and you want a name (suppose 

 you adopt mho) for the unit of conductivity, and 

 call the instrument a mhometer. The rule for 

 resistances in series would be, the sum of the 

 reciprocals of mhos is equal to the number of 

 ohms ; and (or conductivities in parallels, the 

 sum of the reciprocals of ohms is equal to the 

 number of mhos. The number of mhos, or of 

 millimhos, will then measure the number of lamps 

 in the circuit. The domestic incandescent lamp 

 of the early future ought to be, and we hope will 



1 This instrument is represented in Fig. 6 of my patent No. 329 

 of 1858 for "Improvements in Testing and Working Electric 

 Telegraphs." 



