80 



foote: an optical ammeter 



projected directly on the filament C. Another modification of 

 the above method is obtained by reversing the positions of the 

 lamps F and C. The above described principle can be employed 

 for the measurement of alternating voltage, as well as current. 

 In this case the lamp filament must be of very small wire in order 

 that the instrument possess a high resistance. For voltages 

 from 60 to 110 volts a 7.5-watt lamp, which has a resistance of 

 1600 ohms, should prove satisfactory. 



c ur"rp.nt 



Fig. 2. Current-temperature relation for two lamps C and F. If a diffusing 

 screen is used in front of the lamp F, the above curve F refers to the apparent 

 temperature of the screen. 



The apparatus shown in figure 1 may be employed for the 

 determining of a fixed current of any preassigned value. A 

 system similar in principle to this has been used, for determining 

 a fixed temperature but to the writer's knowledge the method 

 has never been employed for current measurements. Suppose 

 that the apparent temperature-current relations of the lamps C 

 and F are represented by the curves shown in figure 2. The 

 two curves intersect at the point P. Hence, when the lamps C 

 and F are connected in series and are caused to glow by either 

 alternating or direct current the two filaments possess the same 



Siemens and Halske French Patent No. 466064. 



