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This one is correct
Jamie is correct, because the mechanical energy is converting to electrical energy.
The relationship between the voltage and the current in the circuit is:

where

is the voltage difference, R the resistance of the circuit (and therefore, the resistance of the battery) and I the current flowing in the circuit. By using I=19 A and

, we can find the internal resistance of the battery:
The general formula for the frequency of the nth-harmonic of the column of air in the tube is given by

where f1 is the fundamental frequency.
In our problem, we have two harmonics, one of order n and the other one of order (n+1) (because it is the next higher harmonic), so their frequencies are


so their difference is

So, the difference between the frequencies of the two harmonics is just the fundamental frequency of the column of air in the tube, which is: