My description of the experiment with the air capacitor
was not complete. There should be an electronic switch
so that charging is done in a different loop that discharging.
The electronic switch would perform the same function as
a mechanical switch but much more rapidly, for example,
every 0.1 ms. Then the calorimeter would measure the
"amount of heat" generated in N discharges, each
delivering only 0.5*C*V^2 of thermal energy.
If f=10 kHz and time is five minutes than N=10000*300.
With C=5000 pF and V=300 volts the deposited thermal
energy (Q=N*0.5*C*V^2) would be 675 J or 161 cal.
Or 645 cal if V=600 volts. Too bad that getting a large
C with an air capacitor is not easy. Ten radio capacitors
of 500 pF in parallel? Why not?
Ludwik Kowalski