228 



SCIENCE-GOSSIP. 



RADIOGRAPHY. 



By James Quick. 

 (Continued from page 204.) 



Electric Lighting Currents. 

 "\ ~\ THEN current is to be utilised from the supply 

 mains, much care must be exercised. If the 

 worker is not fully conversant with electrical matters 

 he should take the advice of an electrical engineer 

 upon the subject, otherwise he may get his induction 

 coil burnt up, or may do other serious damage. 

 Electric lighting circuits are run at different voltages, 



Fig. 14. Resistance Frame, 



varying from 100 to 210, depending upon the 

 company from whom the power is obtained. There 

 are two principal systems of supply, by continuous, or 

 by alternating currents. If the latter is the one at the 

 experimenter's command, it will be necessary for him 

 to transform it into a continuous current before 

 proceeding to its use for his X-Ray work. For this 

 purpose an alternating current motor, coupled to a 

 small dynamo will be required. In either case, 

 though, whether the supply iscontinuous or alternating, 

 the voltage from the mains is much too high for 

 induction coil work. If a coil were connected direct 

 to the mains' terminals, it would be instantly 

 deranged, owing to the excessive current that would 

 pass through. Resistances or rheostats of some kind 

 have therefore to be inserted in the circuit, to cut 

 down the current to the required values. 



Wire resistances are used in a great many instances, 

 and they generally consist of spirals of bare iron, 

 German silver or platinoid wire. These spirals are 

 strung parallel to one another upon a suitable frame, 

 as is shown in fig. 14. The left hand terminal of the 

 frame is connected to the bottom of the left hand 



spiral, and the right hand terminal to the metallic 

 lever of a switch, which can be made to pass over 

 several insulated brass projecting pieces. These latter 

 are each connected to the bottom end of a spiral. By 

 this arrangement the resistance can be varied from 

 that due to one spiral, to the total resistance of all 

 the spirals in series. 



It is difficult to obtain a satisfactory wire resistance 

 arrangement such as the above, when, for experimental 

 purposes, the current is required to be varied from 

 very small to very large values. If the former is 

 required, then the wire must be thin and its total 

 length great, in order to obtain as high a resistance as 

 possible to the current. If, however, a shorter length 

 of this thin wire is taken, so as to increase the current 

 passing through by thus diminishing its resistance, it 

 will become hot and perhaps finally fuse. There is a 

 maximum current that a wire of given diameter can 

 take without becoming unduly heated ; that is, its 

 current carrying capacity is limited. To obtain a 

 large current will therefore necessitate a thicker wire 

 and a correspondingly greater length of it, and a single 

 wire resistance frame, unless it be a large and cumber- 

 some one, cannot be adapted satisfactorily to the 

 extreme values of current. 



A more suitable arrangement is provided by an in- 

 candescent lamp resistance. Suppose a 100-volt 

 incandescent lamp of 16 candle-power (c.p.) is con^ 

 nected direct on to a 100-volt circuit, then the resist- 

 ance of the lamp is such that the current passing 

 through will be about 0.6 ampere. If two such lamps 

 are placed in series the total resistance will be doubled, 

 and therefore, by Ohm's law, the current passing 

 through will be 0.3 ampere, and so on for any number 

 in series. If two lamps are in parallel the total 



Incandescent Lamp Resistance. 



resistance will be one-half that in the case of the single 

 lamp, and the current taken through will therefore 

 be 1.2 amperes. If three are in parallel the current 

 will be 1.8, and so on. If lamps of the same make 

 and voltage, but of 32 c. p., are used, then one lamp will 

 take about 1.2 amperes, and the above reasoning will 

 hold good when two or more are coupled in series or 

 in parallel. The greater the c.p. of a lamp the greater 



