A vertical metal wire segment was used as a breakout point for spark production in Figs. 1 and 4, and a folded piece of metal foil in Fig. 3.

A maximum value $C_\text{max}$ for the charging capacitance can be roughly estimated from the power supply parameters and the repetition rate of 120 Hz as follows. Consider a 60 Hz AC power supply with rated output power $P = V_\text{rms} I_\text{rms}$ and output impedance $Z = V_\text{rms}/I_\text{rms}$, where $V_\text{rms}$ and $I_\text{rms}$ are the maximum root-mean-square output voltage and current. Then $C_\text{max}$ corresponds to the capacitance that charges from zero voltage up to $\sqrt{2}\, V_\text{rms}$ during a quarter cycle, or 1/240 second, at the rated power. From energy conservation this requires $C_\text{max} V_\text{rms}^2 = P / (240~\text{Hz}),$ which gives $$\begin{aligned}
   \label{LTR}  
C_\text{max} = 1/(Z \times 240~\text{Hz}). \end{aligned}$$ For the NST of Fig. 2(e), $C_\text{max} \approx 16.7$ nF. As built, the effective charging capacitance for the arrangement of Fig. 2(a) was 12.1 nF, and of Fig. 2(b) was about 11.8 nF, ignoring the NST filter capacitors (which added $\sim$ 0.24 nF). In practice, the optimum charging capacitance that leads to the most energy per burst without significantly reducing the charging voltage should be 