A calibration resistor and a capacitance load are placed in parallel across the output of a voltage source and a first decay time is determined for the voltage to reach a second voltage from a first voltage after the voltage source is disconnected. The metallic nanoparticles produced by this method can be stably and uniformly dispersed without adding other reducing agent or dispersant. The imaging device has an eye positioned adjacent to the open end of the body. A cover is provided to overlie the tube.