MEASUREMENT OF CAPACITIES IN MOTOR CONTROL 69 



and some of them are complicated and delicate. Fortunately, 

 we have hit upon a sort of universal apparatus which can be 

 used for most of these tests. It consists of an ordinary phono- 

 graph with a series of small attachments. During the year we 

 discovered the surprising fact that the current phonograph 

 motors have an extraordinary high degree of constancy. Under 

 favorable conditions the variation from revolution to revolution 

 of the disc in a good phonograph is less than one thousandth of 

 a second. This is a higher degree of precision than we really 

 need for the measurement of time. We then devised an electro- 

 magnetic marker carrying an ordinary fountain pen and 

 mounted this on an endless screw so that it writes on a piece 

 of white paper placed on the disc plate. With this marker we 

 can connect the signal and the response apparatus so that a 

 mark is made at the time of the stimulus and another at the 

 time of the response. The reading is made simple and quick 

 by the fact that the stimuli can be made at one point through 

 an automatic key, and the time scale consisting of a circle 

 divided into thousandths of a revolution can lie under the disc. 

 In order to get the reading in a given record, all that we need 

 to do is to run a guide lever from the central pivot and over 

 the mark on the record. This will then point to the reading 

 on the scale in terms of hundredths or even thousandths of a 

 second. 



By this means we measure the motor ability, timed action, 

 simple reaction, and complex reaction in the series of motor 

 tests. The same outfit is, however, available for many other 

 measurements not belonging to the series and becomes a cheap 

 and exceedingly valuable general instrument in the laboratory. 



One of my students, Mr. C. F. Hansen, designed a very 

 clever device for the serial action. It consists essentially of a 

 four track contact strip attached to the carrier of a typewriter 

 in such a way that, for every time a key is struck, a new signal 

 will appear. It may be used in connection with any kind of 

 electric exposing apparatus for visual or auditory stimuli. 



For the measurement of endurance we have devised a new 

 ergograph which makes use of the muscles of the forearm in 

 the most natural position of pull. It is built on the principle 

 of the spring dynononieter and makes an automatic ergogram. 



Psychological Laboratories, 

 The State University. 



