ELECTRIC WAVE TELEGRAPHY—FLEMING. 169 
plitudes die away in geometric ratio. Hence the ratio of two succes- 
sive amplitudes or oscillations is constant, and the natural logarithm 
of this ratio is called the decrement. We can determine this decre- 
ment when we know the frequency of the oscillations in the primary 
circuit and the current induced in any secondary oscillation circuit, 
placed near to the first, when the latter is in exact syntonism, and also 
slightly out of syntonism, with the primary. Employing a formula 
of Bjerknes, we can find the sum of the decrements D and d of the 
primary and secondary circuits by the formula 
D+d="(1+8) aes 
where @ is the current in the secondary circuit when it is tuned to a 
frequency n, and A is the maximum current when the secondary 
circuit is tuned to agree with the frequency N of the primary circuit. 
For this purpose I modified the cymometer by including in the bar 
Current in Amyperes A. 
0-7 O08  O9nmN 1:0 11 12 18x06 
Frequency n. 
Fic. 4.—Resonance curve of loosely coupled oscillatory circuit. 
two fine resistance wires, against one of which a sensitive thermo- 
junction of iron and bismuth is attached. This enables me to 
measure the value of the current in the cymometer bar. The process 
of measurement is then as follows: We place the cymometer alongside 
the antenna and slide along the handle slowly, thus altering its time 
period or natural frequency. We observe the current and frequency, 
and plot a curve called a resonance curve showing the secondary 
or cymometer current in terms of the frequency. (See fig. 4.) This 
curve rises to a maximum value, sometimes very sharply, the maxi- 
mum corresponding to the condition of exact syntonism between the 
antenna and cymometer circuits.¢ From the curve we can determine 
“Tf the damping of the secondary circuit is small, as it is in the case of the 
eymometer circuit, then the resonance curve is very sharply peaked or rises 
quickly to a maximum when the primary oscillations are feebly damped, pro- 
vided always that the “‘ coupling” or mutual inductance of the two connected 
circuits is small, 
