Decrement. Measurements Papas, 
The decremeter consists of a coil, a variable condenser, and a radio 
frequency milliammeter or galvanometer connected in series and placed near 
the radiating source. The capacity is varied until the current is a maximum 
or the circuit is in resonance with the source. The capacity of the variable 
condenser is varied until the mean square of the current is reduced to 1/2 
the first value. Then the decrement is calculated. This gives the sum of 
decrement of the source, zerial, and the decrement of the decremeter. This is 
exactly the same as in measuring the resistance of a 1 to 1 transformer circuit 
by introducing resistance in the circuit until the current is made 1/2. The 
value of R introduced is equal to the sum of the resistances in the two circuits. 
This holds if the mutual inductance is large as in a transformer. 
Since d=cT=(R/2L)T, doubling the resistance in either circuit will 
double the decrement of either circuit. 
Thus the introduction of resistance in the decremeter circuit until the 
current in the decremeter is made 1/2 half, the circuit being kept in resonance 
all the time, will double the decrement of the decremeter. Then if 
D,=d,+d= first decrement measurement and D,=2d,+d= second decrement 
measurement with resistance inserted in decremeter circuit. Then d,=D.—D,. 
The decremeter is assumed to be loosely coupled to the aerial so as not 
to affect the aerial circuit. The method is much more simple than that 
usually given, as in formulae 63 and 64, page 94, Radio Instruments and 
and Measurements, Circular of the Bureau of Standards No. 74. This formula 
is: 
6d, 4+0,2— 02 
(oc) 
d—d, 
Where 3?! is the decrement of the aerial, ¢ the decrement of the wave meter 
and 9, the increase of ¢ due to the resistance added which reduces I,? to 1/2 I’. 
6! the decrement of the aerial seems to be given in terms of two un- 
known quantities. The remark is made, “It should not be forgotten that 
these formulae apply only when the coupling is very loose and both decrements 
are small’. This is the condition assumed in the derivation of my formulae. 
The most accurate method of getting the decrement of a decremeter is 
to use a continuous wave current such as is generated in the modern tube cir- 
cuits or wireless telephone circuits. In these circuits the wave is continuous 
or the decrement is zero and the decrement measured is that of the decremeter 
alone. 
This method can be used to determine the decrement of the decremeter 
and thus check the above method. 
The decremeter used contained a 250 milliampere milliammeter whose 
D. C. resistance was 6 ohms. 
When the current in the decremeter was large there was a tendency to 
spark over in the condenser. This brush discharge introduced a resistance 
in the circuit which was more or less variable. This tends to make the 
decrement of the decremeter greater at 200 milliamperes than at 100 
milliamperes. 
Due to the fact that the current is intermittant ina damped wave station. 
This sparking over effect is greater with damped waves than in the case of 
continuous waves. 
The following table gives results with CW and damped waves. Decrement 
of wave meter at wave length indicated. 
15-20320 
