THE WORLD'S ADVANCE 



377 



SELENIUM CELL. 

 (17) A. J. H., Cleveland, Ohio, asks: 

 Q. i. Using a selenium cell with a tele- 

 phone receiver and batteries, will the cell, 

 acted on by light, cause the 'phone to re- 

 spond to the variation in light? Will the pull 

 of an electro-magnet vary as the intensity of 

 the light on the cell? 



A. i. We always regret to have to inform 

 our readers that experimenting with selenium 

 is not a very easy undertaking. At present 

 selenium has only been successfully used in 

 laboratories or under conditions approaching 

 laboratory conditions, and then only by per- 

 sons familiar with its properties. About all 

 you will probably be able to record in the 

 'phones will be a click when the light strikes 

 the cell. Work has been done on "light tele- 

 phones," but, as far as we know, no practical 

 results have been obtained. A spot of light 

 has been caused to fall on a cell, which in 

 turn operates a 'phone. The spot of light is 

 reflected from a mirror controlled from some 

 form of transmitter. In this way the tele- 

 phones have been made to respond to sounds 

 transmitted by means of light. As the current 

 handled by the cell is very small the only type 

 of electro-magnet that would be used is that 

 of a sensitive telephone receiver or something 

 of that type. Selenium cells are made to per- 

 form various "feats" by means of secondary 

 circuits operated from relays controlled by 

 the selenium cells. 



TRANSFORMER HEATING. 



(18) K. T, Nicholson, Pa., asks: 



Q. I. How long should a step-up trans- 

 former for wireless work stand the current 

 on without heating to such an extent as to 

 make it necessary to shut the current off? 



A. i. This all depends on the quality of 

 the transformer. As such a transformer is 

 designed only for intermittent work, a higher 

 heating loss can be permitted than for a 

 power transformer which must remain in the 

 circuit all of the time. We have tested a 

 well-known make of Type E transformer and 

 found that it is capable of running several 

 hours without becoming heated to a danger- 

 ous temperature. The secondaries of some 

 open-core transformers heat very badly even 

 on short runs. 



UNIVERSAL MOTORS. 



(19) F. A. A., Silver City, N. M., asks: 

 Q. i. Is it possible to change a direct-cur- 

 rent motor so that it will operate on alternating 

 currents ? 



A. i. Certain specially designed series 

 motors will operate on both direct and alter- 

 nating currents. Illustrations of this are seen 

 in small sizes of household vacuum cleaners 

 that operate at from 4,000 to 7,000 or more 

 revolutions per minute. As induction motors 

 on 6o-cycle circuits, the maximum speed 

 would be under 3,600 revolutions. A series 

 motor is practically unlimited in speed. Some- 



times these "a.c.-d.c." motors are called "uni- 

 versal" motors. As an illustration of the lar- 

 gest motors with such qualifications may be 

 cited those operating on the N. Y., N. H. & 

 H. R. R., between New York and New Haven, 

 which receive direct currents while on the 

 section involving the N. Y. Central's terminal, 

 but alternating currents when on their own 

 tracks. As a first necessity for such motors, 

 the field magnet must be laminated, and since 

 there is no suggestion that yours is of this 

 sort, the case is hopeless. 



CHANGING DIRECT-CURRENT MOTOR 



INTO ALTERNATING-CURRENT 



MOTOR. 



(20) H. R. B., New Haven, Conn., asks: 

 Q. i. How can I change a direct-current 

 fan motor to adapt it for use on a 6o-cycle, 

 no- volt, 2O-ampere alternating-current circuit ? 

 Motor has two field poles and a 12-slot arma- 

 ture. 



A. I. Unless the field structure is lami- 

 nated you cannot make the change. Even if 

 field is of the desired sort, you may find that 

 motor will hardly run, there being too many 

 turns of wire to permit the necessary current 

 to flow, for with alternating currents there is 

 a choking effect in proportion to the square 

 of the number of turns opposing the current 

 in addition to ordinary resistances and coun- 

 ter electromotive forces. You would have to 

 rewind with coarser wire, say three sizes 

 larger than at present. You can even then 

 expect lively sparking at the commutator. The 

 20-ampere designation is inappropriate, as the 

 motor ought not to require more than one 

 ampere. 



ONE KW. TRANSFORMER. 



(21) S. T., Reedsville, Pa., asks: 



Q. i. I desire to build a one-kilowatt 

 transformer to operate on no volts, ob-cycle 

 supply. Would it be best to build a closed 

 core or an open core transformer? 



A. i. By all means build a closed core 

 transformer. Its cost will be far less and its 

 operation certainly more satisfactory than the 

 open-core transformer. 



Q. 2. What size core should be used? 



A. 2. Sides 6^4" x 2". Use 170 pieces of 

 0.017" silicon steel per side. Ends 524" x 2". 

 Same number and thickness as sides. Use 

 lap method of jointing. 



Q. 3. What size wire should be placed on 

 the primary? 



A. 3. Use no turns of No. n D. C. C. 

 wire. 



Q. 4. What size and how many turns of 

 enameled wire should be put on secondary? 



A. 4. The secondary will require 8,400 

 turns of No. 28. Unless you are familiar with 

 transformer construction we would not ad- 

 vise you to attempt to build the transformer 

 until you had read up considerable data on 

 the subject. 



