790 



Popular Science Monthly 



brated sufficiently to act as a transmitter 

 when connected in on the circuit as shown. 

 Microphones may be made very sensitive, 

 and, when combined with the sound- 

 amplifying contrivance here described, 

 should make exceedingly faint sounds 

 audible. 



The induction coil shown in the micro- 

 phone circuit is not strictly necessary, but 



contact 



Rfcelvef 



Bottery ' 



A microphone made of a cigar box and 

 its wiring diagram to the induction coil 



it will improve the operation of circuits. 

 The primary (big-wire) side goes in the 

 battery circuit, and the secondary (fine 

 wire) side in that of the receiver. An in- 

 duction coil of suitable size may be pur- 

 chased from any telephone company. 



Switchboard Constructed for Use 

 in the Laboratory 



THE illustration shows a simple com- 

 bination of switches that can be 

 used to flow a current of electricity in 

 different strengths for making tests in a 

 laboratory. The switches can be con- 

 nected with the ordinary commercial line 

 carrying 110 volts. Referring to the 



A combination of switches on a base for 

 directing electric current for a laboratory 



illustration, when 1, 4 and 5 switches are 

 in contact, a straight current flows at the 



terminals. Switches 1 and 4 cuts in a 

 series through the coil, and 3, 4 and 5 a 

 series through the light. The switch 2 

 cuts the light into the circuit and 5 cuts 

 the coil into the circuit. 



A common arc light coil is connected 

 to the terminals at the point marked coil 

 and a 110-volt lamp at the place marked 

 light.— T. I. Dekle. 



Alternating Current Charging With- 

 out Rectifier 



IT is not absolutely necessary to have a 

 rectifier with an alternating current for 

 charging storage cells. Imagine that we 

 have a 6-volt storage battery in need of 

 charging and that the house mains pro- 

 vide current at 110 volts A. C. We hook 

 in a bell transformer stepping down the 

 voltage to nine volts as shown by the 

 sketch, and then connect six ordinary dry- 

 cells — new ones — in the secondary cir- 

 cuit, the dry cells being in series with the 

 storage battery to be charged. It is es- 



Tronsformer 



6 storoge bollera 

 Hookup for dry cells with transformer to rec- 

 tify alternating current for battery charging 



sential to connect the carbon pole of the 

 dry cell battery to the positive pole of the 

 storage battery. 



Now to show that the battery is 

 actually being charged: Let us consider 

 conditions when the direction of flow of 

 the A. C. is the same as the direction of 

 flow of the dry cells — for convenience we 

 will call this direction positive. Our 

 transformer gives us 9 volts in a positive 

 direction; the dry cells another 9 positive, 

 while the storage battery gives us nega- 

 tive 6. The algebraic sum or resultant 

 voltage is plus 12. 



Consider now the other or negative 

 alternation. Our dry cells give us plus 

 9; our transformer minus 9 and our stor- 

 age battery minus 6. Resultant, minus 

 6. In other words, on the positive or 

 charging alternation, we have 12 volts 

 acting to force current through the bat- 

 tery, while on the negative alternation we 



