The break of any of the mercury type interrupters when properly adjusted is much quicker than the hammer spring break and gives thicker sparks.
After the mercury has been in use awhile it becomes churned up into small globules of a black color, but may be easily cleaned and restored for use by shaking up with some strong sulphuric acid. Care must be taken that the mercury is perfectly dry and free from acid before replacing in the interrupter.
*Electrolytic Interrupters.*—Fig. 40 shows a diagram of a Wehnelt interrupter. The cathode or negative electrode is a lead plate immersed in dilute sulphuric acid. The anode is a piece of platinum wire placed in a porcelain tube and projecting through a small hole in the bottom, so that only a very small surface of the wire is exposed to the liquid. When a strong electrical current is passed through the acid electrolyte, the current is very rapidly interrupted by the formation of gases on the small platinum electrode. The speed of the interrupter is variable through great ranges by moving the platinum electrode up or down and changing the amount of surface exposed to the liquid. The only disadvantage of this interrupter is that the electrolyte soon becomes heated, and unless the interrupter is provided with a water jacket or some device for cooling, the bubbles of gas do not form freely. A potential of at least 40 volts is required to operate a Wehnelt or other electrolytic break.
[Illustration: Fig. 40. Wenhelt and Simon Electrolytic Interrupters.]
A Wehnelt interrupter may easily be made by sealing a platinum wire in a glass tube. It is well to make several such tubes with the platinum projecting from one-sixteenth to one-quarter of an inch. The different tubes will each have a different speed of interruption, and one should be picked out which seems to be most suitable for the coil upon testing. Connection to the platinum wire is established by filling the tube with mercury and dipping a wire in it.
Fig. 40 also shows a diagram of a Simon electrolytic interrupter. It consists of a vessel containing dilute sulphuric acid and divided into two parts by a thin porcelain diaphragm having a small hole in the center. A lead electrode dips into each of the divisions. The interruption is caused by exceeding a certain current density in the small hole at the diaphragm. Upon the passage of the current the liquid is so heated that it becomes vaporized. The vapor is a poor conductor to low voltages and so the current is broken. Immediately upon the cessation of the current, the vapor condenses and the circuit is established again. This cycle repeats itself with a speed depending upon the size of the aperture and the amount of current flowing.