'158 Prof. E. Taylor Jones on the Potential 



at 1/50 ampere in the primary. The mutual inductance was 

 also determined for various other values of the primary 

 current up to 1*5 ampere, at which value it appeared to have 

 reached its maximum value, viz. 1*07 henry. 



Thus for steady currents and with 1/50 ampere in the 

 primary, the coupling is 



P =M-= 0-87, 



1^2 



and g== l + 2L ^ +Ll =l'04. 



With the lever in the most retarded position and the 

 contact-pieces just separated — in which position of the 

 armature the trailing horns of the core were well clear of 

 the pole-pieces — the coupling was found in the same way, 

 and at the same currents, to be 0*81. 



The above values of the inductances are applicable when 

 the currents are steady, and must not be assumed to 

 hold during the rapid oscillations which take place in the 

 magneto circuits after the interruption of the primary 

 current. 



For the purpose of determining least sparking currents 

 for various capacities, the condenser attached to the armature 

 was disconnected from the primary coil and a separate lead 

 connected with the wire leading to the junction J (fig. 1). 

 A circuit was then formed including the primary coil, 

 a battery, a rheostat, a Kelvin graded galvanometer, and a 

 mercury-oil interrupter worked by hand, the circuit being 

 completed through the frame of the magneto and the 

 armature core. Connected directly in parallel with the 

 interrupter was a mica condenser of variable capacity 

 ranging from 0*001 to 1 microfarad. 



After numerous trials it was found advisable to remove 

 the high-tension connecting rod and its carbon collecting 

 brush, and to replace them with an insulated copper wire 

 pressed into contact with the collecting ring and connected 

 with one side of the H.T. spark-gap ; also to insert a 

 connecting wire between the armature core and the frame 

 to ensure good electrical contact. The H.T. spark electrodes 

 consisted of two cylinders terminating in a plane and a 

 spherical cap (radius about 8 mm.), the gap between them 

 being finely adjustable. With these changes, and with 

 clean mercury under paraffin oil in the interrupter, the 



