PROBLEMS IN THE DESIGN OF SPECIFIC INSTRUMENTS 



95 



collected by Peterson (62) for different air- 

 craft compass designs also support the air- 

 craft reference principle. Although Fitts 

 and Jones anticipate that this principle may 

 not apply to the design of all orientation 

 instruments, they look for research to 

 demonstrate where these exceptions lie. 



The design of a clock face to be used in 

 reading time on a 2400 military scale was 



on the poorest designs, C and I. Average 

 time per reading, including time for the 

 subject to write his answer, did not vary so 

 much — from less than 6 to about 7 seconds 

 for the different dials. Specific comparisons 

 within the study showed that it was desirable 

 (1) to graduate every minute, not just every 

 five, (2) to number the minute scale at five- 

 minute intervals, (3) to number all hour 



E F K 



Fig. 12. Experimental designs used in the study of clock dials for reading in the 2400 military time 

 system. From Grether (29) 



investigated by Grether (29). By using 

 pencil-and-paper test materials he was able 

 to explore the relative merits of eleven differ- 

 ent clock designs in reasonably short order. 

 These designs are shown in Fig. 12. Perhaps 

 the most revealing result of this experiment 

 was the wide range of error frequencies found 

 for the different designs. There were only 

 4% incorrect readings on the best clock, de- 

 sign J, but more than 20% incorrect readings 



positions, (4) to use a 24-hour, single-revolu- 

 tion scale, rather than a 12-hour, two-revolu- 

 tions scale, (5) to start and end the hour scale 

 at the bottom of the dial, (6) to start and end 

 the minutes scale at the top of the dial in the 

 conventional way. The dial which com- 

 bined all these features is dial J in the figure. 

 It is clear that problems in the design of 

 other specific instruments will yield to experi- 

 mental investigation in the same manner as 



