Cady and Arnold — Electric Arc. 409 



§38. It seemed desirable to test the continuity of the arc by- 

 some method less likely to set up disturbing oscillations. 

 When the condenser-bolometer system was connected in paral- 

 lel with a non-inductive resistance which was in series with the 

 arc, no deflection whatever could be observed. But the con- 

 nections shown in tig. 6 were found to give excellent results. 

 A is the arc, R a regulating resistance, L the primary of a 

 large induction coil whose iron core was removed, consisting of 

 200 turns of heavy wire, with a self-inductance of about 0*0012 

 henry. In parallel with L are the capacity C of 3 - 6 m.f., the 

 bolometer wire B and the switch K. 



When A" is closed, the bolometer is not heated as long as the 

 arc current remains constant. But any fluctuation of current, 

 producing a varying potential difference between the terminals 

 of _Z, sends an alternating current through the shunt circuit 

 and causes a deflection of the galvanometer in the bridge. 

 Correction was made for a slight deflection due to the passage 

 of the commutator bars under the brushes of the generator. 



With the iron arc in air, deflections of the order of 30 div. 

 w r ere found at the beginning of the second stage, which reached 

 a maximum while the arc was rotating at about 2 amp. and then 

 decreased again. There was no deflection on the first stage 

 after the effect of the commutator had been eliminated. 

 When a singing arc was produced on the first stage by con- 

 necting a second capacity and inductance in parallel with the 

 arc, a large deflection resulted. 



Sahulka* finds that a self -inductance connected in series 

 with a singing arc oscillates with a frequency of its own, and 

 not simply with the arc frequency. This point we have not 

 tested. 



A simple method of comparing frequencies is to insert an 

 iron core in the coil Z, fig. 6. When this was clone, it was 

 found that for a singino- arc on the first stage the insertion of 

 the core produced no perceptible effect on the galvanometer 

 deflection. With the arc on the second stage, not singing, the 

 iron core increased the deflections about tenfold, while the 

 fluctuations due to the commutator of the generator gave a 

 deflection that was multiplied about thirty times by the inser- 

 tion of the core. Thus the oscillations due to the volatilization 

 of the anode on the second stage are intermediate in frequency 

 between those due to the commutator and those of the singing 

 arc. The last are so rapid that the self-inductance of the coil 

 is no longer increased on inserting the iron core. 



Tests were also made with the iron arc in nitrogen at atmos- 

 pheric pressure. The irregular sputtering condition of the 

 arc gave rise to a continual heating of the bolometer, which 

 *Ecl. Electr., 1, 396, 1907. 



