the High-Tension Magneto. 291 



5. The arrangement just described, in which the oscil- 

 lations are excited by breaking a current in the primary 

 while the secondary potential is measured, is suitable only if 

 the period of the slower component of the oscillations is 

 required. But if, as in the determinations of c and c', the 

 period of the more rapid component is required, the method 

 is not available; for if, as is necessary in those determinations, 

 either Ci or C 2 is made very large, the amplitude of the rapid 

 component in the secondary potential becomes very small 

 compared with that of the slow component. When C 2 is 

 made large, and c 1 measured, the difficulty can be overcome 

 by measuring the primary, instead of the secondary, 

 potential ; in such circumstances the amplitude of the more 

 rapid component of the primary potential is the greater. 

 Accordingly all that is necessary is to connect the measuring 

 apparatus to the primary circuit instead of to the secondary 

 potential*. 



But when C^is large the slower component has the greater 

 amplitude in both circuits, if the oscillations are excited by 

 breaking the primary current. If, however, they are excited 

 by starting the primary current, the amplitudes of the two 

 components in the secondary circuit are the same. Ac- 

 cordingly, when c was to be measured, contact 1 was placed 

 as a shunt across the primary terminals, so that the current 

 started when it was broken f. The oscillations now take 

 place with the primary circuit closed, i. e. with Ci infinite. 

 But since it is necessary for a measurement of c that C 2 should 

 be very large, no limitation is thereby placed on the appli- 

 cation of the method. 



Experimental Results. 



5. The first experiments were made on a Thomson-Bennett 

 magneto, Type AD 4C ; this machine has a laminated core 

 but unlaminated pole-pieces. The armature was fixed in the 

 position of maximum inductance, and the terminals of the 

 primary circuit disconnected from the condenser and brought 

 outside the machine. The air-condenser used for measuring 

 the secondary potential was always connected to the secondary, 



* Since the primary potentials are much less than the secondary, it is 

 convenient also to increase the sensitivity. This is easily done by 

 increasing the condenser C. 2 hy the substitution of a leyden-jar of about 

 2000 mmf . for the air-condenser. So long as this condenser has a capacity 

 small compared with that of the rest of the circuit, its exact value is, 

 of course, immaterial, and affects only the calibration constant. 



t In order that the resistance of the shunt might be small compared 

 with that of the primary, a few ohms extra resistance had to be inserted in 

 the primary circuit. 



