456 KNIGHT DUNLAP 



Action time between 20 sigma and 90 sigma may readily be 

 secured, or if pathological or anticipatory reactions are to be 

 measured, the action may be made within the limits of 20 and 

 50 sigma. 



For other types of stimulus, variations in the above scheme 

 are required, but the balanced relay will in almost all cases be 

 the main accessory. For the reaction circuit, no special appa- 

 ratus or wiring are needed on account of the balanced circuit, 

 since the reaction circuit does not have to be reclosed. 



GENERAL CAUTIONS 



The Johns Hopkins chronoscope, like the Hipp, requires careful 

 testing to assure balance of the magnet latencies and of the 

 armature slips, but once set up and thoroughly tested, no further 

 checks are needed unless the instrument is allowed to get badly 

 out of condition. We have found that chronoscopes which were 

 properly balanced, have become unbalanced in shipment, either 

 through bending of the armature disc or other injury. The 

 anterior clutch magnet (or in later designs, the magnet-core) 

 has so far been made so that it can be rotated by hand ; an adjust- 

 ment which is not needed in most work, and can be dispensed 

 with. This magnet bearing must be tight enough so that the 

 magnet will not creep under the torsion of the armature disc 

 at stopping. Oil on the magnet face or armature disc will cause 

 clinging. The clutch must be kept clean. 



The chronoscope should be set on a square of thick harness 

 felt. If the tuning fork is in the reaction room, it also should 

 be set on harness felt. The hum of the apparatus is then reduced 

 to a negligible intensity. In using voice keys, they should be 

 supported on a heavy, solid table, to avoid their being susceptible 

 to the jar of the master key and movements of the operator's 

 hand. 



To remove the most probable source of error in the Johns 

 Hopkins chronoscope, I propose to replace the friction clutch 

 by a modification of the Hipp toothed clutch. It seems advis- 

 .able also in the next model to gear back the register hand so that 



