The Birmingham age-herald., 1922-06-04
VAR|OME TER IS Fb kHHNMMN VWEVb USEFUL PART
 IN RECEIVER
Is Easily Made and Permits Inductance to Be Varied Without Any Break in
 Continuity of Circuit

One of the most useful instru ments in the radio receiving set il the variometer. It is most efficien on shortwaye set, and as the broad casting stations are all operating by government decree, upon 360 me ters, the variometer finds a yer, large sphere of usefulness in the ra diophone system. It affords a mean! of getting continuous variation o the inductance in the radio frequen cy circuits Without the disadvantag, of sliding contacts or the multipoin switches that are used in other forms of irductance. It also has tn, advantage that 1t can be readily ant simply constructed.
 Another Important point in its ia yor, when used with rne vacuum tube sets, Is the fact. that its pc culiar nature permits the variation of it's inductance without any break in the continuity of the circuit. This is very important in vacuum tube work, because with the multipoin' switches they are very carefully de signed and constructed, the circuit is apt to be broken while the ad- justments are being made, and ir some cases this will tend to paralyzs the vacuum tube.
 Consists of stator and Rotor
 A variometer Is constructed in two sections--one known as the stator and the other as rotor. The stator as its name implies. Is a stationary winding in a suitable form. while the rotor is winding upon coil which rotates upon it's axis inside of the stationary winding.
 The operation of the variometer Is as follows. If the rotor is turned in such manner that It's plane coin- cides with the plane of the station ary coil, the current will / circle through both windings in the same direction and their magnetic fields will add to each other. The self induction of the variometer is then at It's maximum point.
 Should the rotor be turned com p,etefy around. the current will then flow through the two coils in oppo- site directions and the result will be that the two magnetic fields will op- pose each other and the self-induc- ton of the variometer will be at the minimum. or practically zero. Of course, If the rotor coil Is at any other angle, we obtain varying de- grees of self-induction, and 1t Is by this means that tuning is affected with the variometer.
 variometers Standardized
 In the past couple of years. the variometers used in receiving cir cults have become more or less standardized, and the forms upon which the two windings are laid can be obtained ready made by the radio fan. They invariable coffre In three pieces-two of the pieces represent ing sections of the stator and the third piece being the rotor. The latter is invariably made in ball form.
 For the novice who desires to con struct his own variometer without obtaining any of these standard parts. the following instructions should be followed. Obtain two cardboard tubes--one inches in di- ameter, both being inches in length. Upon these wind single layers of No 24 s. c. c. wire. The winding will not be continuously made, because in the middle of each tube a space of one-duarter inch must be left so that the shaft which revolyes the rotor can be passed through both coils. The same amount of wire should be placed on each tube. com pletely filling the tube with the ex- ception of the one-quarter inch space in the middle and a slight space on each edge.
 The outer tube can be mounted on a wooden frame after hole has been drilled sufficiently large enough to take the piece of quarter inch round brass rod 8 inches in length. The inner tube should be fastened to this brass rod so that 1t will revolve with it.. One end of the rod will have an adjusting knob attached to it. A scale can be added If desired. The turns should be thoroughly shellacked after the winding is complete. in order to pre- vent that from loosening up.
 Standard Blocks Can He Used
 With the standard wooden blocks or composition blocks that are sold for variometer purposes all that will be necessary is to wind the spaces left for winding with No. 22 D. C. c, wire, so that the entire space Is filled up. At the first glance it will look as though the task of wiring the stator section wfll be extremely diff ficult, but if these instructions arc followed it will not be a very hard task.
 Cover the sections of the stator with a coat of sheilac and before this dries put your winding upon it. seeing that the winding Is snug and tight. After this is completed cover with another coat of shellac and force the windings into position and then allow it to dry.
 In assembling Che variometer. the end of the stator winding will be carried on and connected to the be- ginning of the rotor winding. In thfa case, however, flexible connection of sufficient length must be left be- tween the two coils in order to per init the rotor to be turned com- pletely around without breaking the connection. Another way is to use the brsss shaft as means of con meeting tho two coils together. SC that the rotor coil can be spun com pletely around.