72 ROBERT M. YERKES AND JOHN B. WATSON 



(It is assumed that the current used will be D. C, ranging between 

 io8 and 120 volts.) This voltmeter will show the general steadi- 

 ness of the power-house or city circuit. The city current should 

 be used at the time of day when it is most steady. The ammeter 

 should read 0-25 amperes. These instruments need not be of 

 expensive type. Jas. G. Biddle, Philadelphia, has satisfactory 

 instruments, listed as the Keystone type C. voltmeters and 

 ammeters. The cost is about $15 each for voltmeter and ammeter. 



In using the arc with the Bausch and Lomb adjustable rheostat 

 admitting the maximum of current it will be found that the 

 amperage is at first rather high, but that as the lamp and resis- 

 tance box gradually heat up the amperage drops to 23 or less. 

 This increased resistance soon reaches a maximum, however, 

 if the room is well ventilated. The arc will thereafter burn 

 steadily within fixed limits of amperage. 



It is advisable to take off the casing of the Bausch and Lomb 

 lamp and to study the mechanism in action. It comes already 

 adjusted but there is a small " vibrator " or interrupter at the 

 bottom of the lamp which may need adjusting. This is the 

 chief source of trouble in the lamp, but it is not serious. The 

 small feed wires connected with the negative and positive carbon 

 receptacles are too small as they come with the lamp, and are 

 likely to burn out, especially if the carbons are not screwed in 

 tightly. It is desirable to replace these with slightly larger 

 wires if this mishap occurs. The lamp is supplied with both 

 horizontal and vertical adjustments. The positive carbon is 

 put in line with the optical axis of the system, and leveled with 

 an ordinary spirit level. 



The Nernst filament. The Nernst glower is the most satis- 

 factory form of source where a spectrum of medium to low 

 intensity can be used. It is the only source suitable for stimulus 

 limen (R. L.) and stimulus difference (D. L.) work. A card 

 containing 12 filaments, .8 amperes, no D. C, may be pur- 

 chased from the Nernst Lamp Company, Pittsburg, Penn- 

 sylvania, for $3. In addition to the filaments a small " ballast " 

 (resistance) must be burned in series with the filament. This 

 is supplied for 50 cents. In doing quantitative work this fila- 

 ment should be burned in the circuit with a volt- and ammeter. 

 The voltmeter just described is adequate but since the amperage 

 is small, an ammeter reading o-i (or 0-5) should be used; this 



