This video shows a high voltage measurement using sphere gap method. The measurement method and voltage values corresponding to certain gap distances can be found in Standard IEC 60052. Because I just came to know about this, so I am curious how it really works and how accurate it is.

In the video, we can see the high voltage source (can supply up to 20 kV) and a pair of metallic balls. The distance between the balls at which the sparks start to take place corresponds to a certain voltage. According to the Standard, for a diameter of 5-cm balls (which I have), the following relations apply:
You may have heard that the voltage breakdown of air is about 30 kV/cm. This is referred to this sphere gap method. In fact, if the electrode is not spherically-shaped, i.e pointed tip, the electrical potential becomes very concentrated. As a result, a 30 kV voltage can generate spark at distances more than 1 cm.

Well, although my setup does not strictly follows the Standard, but I can say that the measurement results are quite close comparing against the high voltage source, which is my reference in this measurement.

On top of that, it is fun to witness the sparks!! 🙂