The Application of Battery Potential

Radio Record, Volume II, Issue 36, 22 March 1929, Page 31

The Application of Battery Potential

T has been frequently remarked that a ecarborundum is improved by the addition of battery potential. In Vo. 2, No. 25, a method was given whereby this may be applied. It now remains to explain the action of this potential, and to give a few construc: tional hints as to how the potentiometer may be constructed. Calculating Resistance. "THE carborundum crystal, like a valve, has a curve. For example, if 0 volts are applied, about 400 micro amperes will be passed, but with the application of 3 volts, 3000 will be passed, but at this point distortion would take place exactly in the same method as it would take too little grid bias were applied to the valve, ‘The point, then, is to find out exactly how much between 1 and 8 volts is to be applied. This is the function of the potentiometer, for each crystal differs from another. Potentiometers are usually fairly expensive, but the average constructor can make one for himself very easily. Resistance wire is necessary, and this ean be obtained from several dealers in New Zealand. This resistance wire is like ordinary wire made up in several gauges, the range being usually from 36 to 41. According to the make of wire, resistance per yard, or as it is usually expressed, per thousand yards, varies. For example, nickel chrome wire of 41 gauge has a resistance of about 92,000 ohms per thousand yards, that, is, 92 ohms per yard. Another type of wire at, say, 40 gauge, may have a resistance of 32,000 ohms per thousand yards, that is, 32 ohms per yard. The usual resistance for a potentiometer for the type required with a carborundum, is about 400 ohms. Tables are usually given showing the size of wire and resistance per thousand yards at different temperatures. Divide the total number of ohms by 1000, and this will give the resistance per yard. If this figure is divided into 400 (or whatever the resistance required may be) the number of yards of resistance wire will be arrived at. Take, for example, gauge?41 nickel chrome resistance wire, which is obtainable from Messrs. Lawrence and Hansen, Wellington. The resistance of this is approximately 92 ohms per thousand yards, that is, 92 ohms per yard. 400 divided by 92 comes out to approximately 5 yards. 40 gauge "Ad- vance" wire, supplied by Messrs. Johns, Lid., Auckland, has a resistance of about 32,000 ohms per thousand feet, or 32 ohms per foot. This would indicate that 12 feet are required for a resistance of 400 ohms, Naturally, one would expect the "Ad« vance" to be moms amnansime,

Construction of a Potentiometer. To complete construction of a poten- : tiometer, take a good non-conduct-ing substance similar to that used as a base for resistance wire in a potentiometer or rheostat. Cardboard would do if it were covered with a coat of paraffin wax, or given several coats of shellac. The width should not exceed 4 an inch, while the length will be determined by the gauge of wire used, and the resistance to be obtained. Secure one end, and commence winding, keeping each turn free from its neighbour, unless the wire is covered. When the total’ length, has been wound on, secure the end, and arrange on a suitable mount. The ends should be passed into terminals, and a slider rigged up. The varieties of sliders are so numerous that any constructor will be able to make one from the material on hand. Meccano sets come in handy in this respect. Now attaching one end of the potentiometer to a 38-volt battery, and the slider to the telephones, the battery is eonnected to the carborundum,