Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

When the alternate sheets of tin foil and paper have all been assembled, the condenser is warmed so as to soften the paraffin. It is then placed between two flat boards and subjected to great pressure in a letter press or a vice. The capacity of a pressed condenser is often several times that of a condenser of the same dimensions but not pressed.

Mercury Interrupters.—The mercury turbine interrupter is one of the most convenient and successful breaks in use. The construction is such that a stream of mercury is made to play against a number of saw shaped metal teeth. A spiral worm terminating in a nozzle at the top is rapidly revolved by an electric motor. The lower end of the tubular worm dips in a mercury reservoir, so that when the spiral is revolved the mercury is caused by centrifugal action to rise in the tube and be thrown out in the form of a jet at the upper end. When the revolving jet strikes one of the metal teeth, the circuit is closed and the current flows from the mercury jet into the teeth. When the mercury jet passes between the openings between the teeth, the circuit is interrupted. By raising and lowering the saw teeth so that the mercury strikes either the lower or upper part of them, the ratio between the make and break may be made smaller or larger. By regulating the speed of the motor driving the jet, the number of interruptions may be varied from 10 to 10,000 per second. The bottom and sides of the mercury reservoir are ribbed to prevent the mercury from attaining a rotary motion.

A somewhat simpler and more easily constructed type of mercury interrupter consists of a hard rubber disk having a brass rod running through from the periphery to the center, where it connects with the shaft. The lower edge of the disk dips at an angle in a mercury bath and is rapidly revolved by an electric motor. When the rod is under the surface of the mercury, the circuit is made through the mercury to the rod. The circuit is broken when the rod is above the surface. The mercury is covered with a layer of alcohol, which prevents excessive sparking and makes a quicker break. An interrupter of this kind when run by a motor of the magnetic attraction type is exceedingly simple.

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.

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.