Radio Broadcast (Nov 1926-Apr 1927)

190 RADIO BROADCAST ADVERTISER Dubilier Ducon Price $1.50 Aerials have In the old days, when radio was new, the fan was known by crazy festoons of wire that decorated his housetop or yard. These were the old fashioned aerials, and no one has forgotten all the grief they caused. Modern radio may use the hidden loop, or the short indoor aerial. But there is a better way. The Dubilier Ducon enables you to use the complete wiring system of your house without risk, and with better results than most outdoor aerials give. You simply screw a Dubilier Ducon into any lamp socket, and connect it with the antenna binding post of your set. You will find that it increases selectivity — especially in crowded neighborhoods, and will reduce "static" in the summertime. Try a Dubilier Ducon on your set tonight. They are sold by all good dealers on five days' trial for $1.50. Dubilier + CONDENSER AND RADIO CORPORATION The Radio Broadcast LABORATORY INFORMATION SHEETS TNQUIRIES sent to the Questions and Answers department of Radio Broadcast have until recently been answered either by letter or in " The Grid." The latter department has been discontinued, and all questions addressed to our technical service department are now answered by mail. In place of " The Grid, " appears this series of Laboratory Information Sheets. These sheets contain much the same type of information as formerly appeared in " The Grid," but we believe that the change in the method of presentation and the wider scope of the information in the sheets, will make this section of Radio Broadcast of much greater interest to our readers. The Laboratory Information Sheets cover a wide range of information of value to the experimenter, and they are so arranged that they may be cut from the magazine and preserved for constant reference. We suggest that the series of Sheets appearing in each issue be cut out with a ra\or blade and pasted on filing cards, or in a note book. The cards should be arranged in numerical order. Several times during the year, an index to all sheets previously printed will appear in this department. The first index appeared last month. Those who wish to avail themselves of the service formerly supplied by " The Grid," are requested to send their questions to the Technical Information Service of the Laboratory, using the coupon which appears on page 218 of this issue. Some of the former issues of Radio Broadcast, in which appeared the first sets of Laboratory Sheets, may still be obtained from the Subscription Department of Doubleday, Page Sr Company at Garden City, "New York. Radio Broadcast Laboratory Information Sheet December, 1926 Trickle Chargers different types available TT HAS been customary in general to operate a radio receiver from a storage battery having a very large capacity. However, during the last year or so there has come into rather common use the combination of a storage battery with a trickle charger. This combination consists of a small storage battery which is directly connected to the trickle charger. The trickle charger, connected to the a.c. mains, serves to keep the battery in a constantly charged condition. There are several types of rectifiers which have been used in trickle chargers. In the bulb type of rectifier, with which we are all familiar, a small vacuum tube is used which rectifies the alternating current and supplies it to the battery. This type is more familiarly known as the Tungar or Rectigon trickle charger, and is very satisfactory and dependable. The second form of rectifier is the electrolytic type which consists of two electrodes suspended in an electrolyte. It is very simple to construct and works very satisfactorily. It is probably more efficient than the above type since it does not require any energy to light a filament. The third type, which has only recently come into prominence, uses a crystal. We are all familiar with the crystal detector used in a radio receiver which functions to rectify the small radio frequency currents, and since the trickle charger need only supply a small current it seems quite possible to use a number of crystal detectors in parallel. Several models using this system are now on the market. The battery used in conjunction with a trickle charger need not be very large since, under normal operation, it need only be large enough to operate a receiver for one day, after which it may immediately be charged. However, it is wise to use a fairly good size battery — a unit having a capacity of about 40 ampere hours should give quite satisfactory operation. With such a battery in use, it will be possible to operate the receiver for several days without charging, and in this way preparation is made for any emergencies that might occur. When the storage battery is operated in conjunction with a trickle charger the only attention required is to see that the water in the battery is kept above the plates. It will be best to examine the battery about once a month. The rate of charge should be adjusted so as to keep the battery fully charged. This means that, when the charger is first purchased, frequent hydrometer readings should be taken to determine the condition of the battery. If the battery begins to run low the rate of charge should be increased; if the battery gases considerably when on charge, it is an indication that the battery is full and it will then be best to reduce the rate at which the trickle charger supplies current, so as to prevent excessive charging. Radio Broadcast Laboratory Information Sheet December, 1926 Hard and Soft Vacuum Tubes SOFT TUBES FOR DETECTOR USE TF A vacuum tube, during manufacture, has left within its bulb a small amount of gas, or if a small amount of gas is introduced into the tube, it is known as a soft tube. If every particle of gas is removed, the tube is known as a hard tube. Soft tubes are particularly suited for use as detectors. They generally require somewhat critical adjustment of the filament and plate voltages but, once these potentials are found, the soft tube makes a very sensitive detector. Recently some progress has been made in designing sensitive detector tubes, such as the 200-A, which do not require especially accurate adjustment of the operating voltages. The action of a soft tube depends upon the fact that, at certain critical voltages, the gas in the tube is practically in a state of ionization. Then, when a signal is impressed on the tube, the plate current is caused to change, due to the increase in the flow of electrons from filament to plate and due to the increase in the number of ionized gas particles which also flow to the plate. The plate current is therefore increased by these two effects, so that the total change in plate current is greater than it would be if no ionization occurred. When a soft tube is used for a detector, the grid return must connect to the negative end of the filament instead of the positive, as is done when a 201-A tube is used for detection. Therefore, when changing over from a 201-A to a 200-A. be sure to alter the connections so as to have the return connected to the negative filament. Hard vacuum tubes are generally used as amplifiers, and structurally, are the same as the soft tubes, the only difference being that they have no gas content. Amplifier tubes do not require any criti 0.00025 " 6 cal adjustment and will operate on from 40 volts up to the maximum that can safely be placed on the tube. A diagram of connections of a 200-A detector tube is given herewith. •ft Examined and approved by Radio Broadcast