1905.] Oscillations into Continuous Currents. 477 



glow lamp contains a pair of carbon filaments or a single filament and 

 a metallic plate sealed into the bulb, the vacuous space between 

 possesses a unilateral conductivity of a particular kind when the 

 carbon filament, or one of the two filaments, is made incandescent. I 

 have quite lately returned to this matter, and have found that this 

 unilateral conductivity exists even with alternating currents of high 

 frequency and is independent of the frequency. Hence, in a suitable 

 form, it seemed possible that such a device would provide us with a 

 means of rectifying electric oscillations and making them measurable 

 on an ordinary galvanometer. The following experiments were, 

 therefore, tried : 



Into a glass bulb, made like an incandescent lamp, are sealed in the 

 ordinary way two carbon filaments, or there may be many filaments. 

 On the other hand, one carbon filament may be used and a platinum 

 wire may be sealed into the bulb terminating in a plate or cylinder of 

 platinum, aluminium or other metal surrounding the filament. It is 

 preferable to use a metal plate carried on a platinum wire sealed into 

 the glass bulb, the plate being bent into a cylinder which surrounds 

 both the legs of the carbon loop. The diagrams in fig. 1 show various 

 forms of the arrangement. Diagram a shows a bulb with a single carbon 

 filament surrounded by a metal cylinder, b shows one with two carbon 

 filaments, and c a carbon filament and two insulated metal plates. The 

 ends of the carbon filament which is rendered incandescent are marked 

 + and - and the terminal of the other electrode of the valve is 

 marked t. The bulb must be highly exhausted to about the pressure 

 usual in the case of carbon filament incandescent lamps, and the metal 

 cylinder or plate must be freed from occluded air. 



Suppose that we employ such a bulb containing one carbon filament 

 surrounded by a metal cylinder (see a, fig. 1). The filament may be 

 of any voltage, but I find it most convenient to employ filaments of 

 such a length and section that they are brought to bright incan- 

 descence by an E.M.F. of 12 volts. The voltage and section of the 

 filament should be so arranged that the temperature of the filament 

 corresponds with an "efficiency," as a lamp-maker would say, of 

 2-75 or 3 watts per candle. The filament is conveniently brought to 

 incandescence by a small insulated battery of secondary cells. A 

 circuit is then completed through the vacuous space in the bulb between 

 the cylinder and the filament by another wire which joins the external 

 terminal t of the metal cylinder and that terminal of the carbon 

 filament which is in connection with the negative pole of the heating 

 battery. In this last circuit is placed a sensitive mirror galvanometer 

 of the movable needle or movable coil type, and also a coil which may 



vol. 13, Part LXXXIV, p. 45, Friday evening discourse on February 14, 1890, 

 " Problems on the Physics of an Electric Lamp," when this unilateral conductivity 

 was experimentally shown. 



