KENNELLY AND UPSON— HUMMING TELEPHONE. 



343 



The results recorded in die last two columns of Table II. are pre- 

 sented graphically in the target diagram of Fig. 7. The square in- 

 cludes all the good instruments and none of the bad. The mean of 

 the good transmitters is indicated by the solid black circle. 



Table II. 



Tabic of Comparative Hum-Extinguishing Resistances for 12 Good and 12 



Defective Transmitters. 



Observation Series 6. Effect of Varying tJie E.M.F. in the 

 Primary Circuit. — Among so many variables and variations as are 

 displayed in preceding diagrams, it is comforting to find one variable 

 w^hich produced relatively little eflfect within certain practical limits. 

 Fig. 8 shows the frequencies and primary currents for tube-lengths 



