934 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



heavy copper block. The thermocouples are Chromel-Alumel wires of 0.012 

 cm diameter and one couple of each pair is welded to its platinum wire 2.0 

 cm from the point where the wire is clamped in its copper block; the other 

 couple of the pair is electrically insulated by a glass coating and is buried 

 in a deep hole in the block. The electrical contact is made between points of 

 the platinum wires a little beyond the welded thermocouples, and opening 

 and closing of the circuit is achieved by striking one of the platinum wires 

 beyond the point of electrical contact with the insulated armature of a 

 speaker unit vibrating at 60 cycles. Each heavy copper block with its plati- 

 num wire is mounted on a cantilever bar, the end of which can be moved 

 by a screw to permit fine adjustment of the contacts. Adjustment can be 

 made also by varying the voltage supplied to the speaker unit. 



In order to minimize thermal disturbances this equipment is mounted 

 on a heavy steel base, and the speaker unit, which dissipates about 0.01 

 watt during operation, is thermally insulated from the contacts by three 

 concentric heavy aluminum covers each in very good thermal contact 

 with the steel base. All of this equipment is covered by a silvered bell jar 

 of 21 cm inside diameter. An aluminum covered Celotex housing surrounds 

 the bell jar and the thermocouple galvanometer, except for a small glass 

 window for reading the galvanometer. The galvanometer light is turned 

 on for only about one second at the time of each reading. The experiments 

 are made in a constant temperature room. 



All of the significant tests consist in measurements of the heat flow along 

 the platinum wires when they are brought together 60 times per second 

 discharging at each closure a capacity of 10~^ f charged to a potential of 40 

 volts. At this potential an arc occurs between clean platinum electrodes if 

 the circuit inductance is less than about 10~^ h, but there is no arc if the 

 inductance is much higher than this.^ If the electrodes are operated in the 

 presence of any one of various organic vapors they become coated with car- 

 bonaceous material and arcing then occurs at every closure even when the 

 inductance is quite high,^ Measurements have been carried out under three 

 different experimental conditions: (1) clean electrodes with a circuit induc- 

 tance of 0.05 X 10~* h and an arc at every closure, (2) clean electrodes with a 

 circuit inductance of lOX 10"® h and no arcing, and (3) electrodes slightly 

 carbonized by d-limonene vapor with a circuit inductance of 10 X 10~^ 

 h and an arc at every closure. The condition of arcing on every closure, or of 

 complete absence of all arcing, was readily determined for each experiment 

 by continuous oscilloscopic observation.* The potential of 40 volts was 

 chosen as the highest at which there is never a second arc (in the reverse 



*See reference 1, Fig. 1. 



