BALANCED MAGNET CHRONOSCOPES 449 



ture speed is 5 rotations per second, the measuring unit of the 

 dial is 2 sigma which is small enough for all purposes and 

 the motor is easily started. If a rectifier is used with the 60 

 cycle current the impulses are reduced to 60 per second, the 

 measuring unit is one six-hundredth of a second, the motor 

 starts easily and runs well without heating. I have found the 

 small size "Tungar" rectifier, General Electric Company, very 

 satisfactory when connected directly in the circuit without 

 resistance. 



A small transformer may be used directly in the circuit, reduc- 

 ing the voltage to the point required to force the proper amount 

 of current through the field coils. This has the advantage of 

 economizing current, a large proportion of which is wasted in 

 the resistance of the above described arrangements. The trans- 

 former should step the voltage down one-half; i.e., should reduce 

 the 120 volt potential to 60, or the 110 to 55. 



It is necessary to bear in mind that the nominal cycles per 

 second of a lighting or power circuit are only approximate, and 

 that there is a slight variation from the standard from time to 

 time. The alternating current can be used for demonstration 

 purposes, and for practise work where accuracy is not essential, 

 without check. But, for research purposes it is necessary to 

 use a frequency-meter from time to time to determine the actual 

 frequency; or else to obtain from the power house data on the 

 diurnal variations in frequency. In many situations it will be 

 found that the variations at the hours at which the apparatus 

 is used are small enough to be neglected in the calculations. 

 Since the frequency determines the value of the units of measure- 

 ment of the chronoscope, reaction times obtained while running 

 the motor on a current of uncertified frequency are not worth 

 much. 



For the above reasons, it is recommended that the motor be 

 operated on direct current, with tuning fork interruption where- 

 ever possible. 



In using direct current in the motor circuit, an electric fork 

 is necessary to interrupt the circuit periodically. If the 120 

 or 110 volt current is used, a voltage-reducing lamp board should 



PSYCHOBIOLOGY, VOL. I, NO. 6 



