406 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 17 



sired average value by adjusting the ratio of the length of time the 

 one rate is being supplied to the length of time the other is being 

 supplied. In the case of a galvanometer having mechanically operated 

 contacts, the circuit may only be closed at particular instants sepa- 

 rated by arbitrarily fixed time intervals. Consequently, the ratio 

 referred to must be approximated by a fraction whose numerator and 



Time 



Fig. 2. Diagram to illustrate the periodic fluctuations in temperature of a furnace con- 

 trolled by a galvanometer having mechanically operated contacts. 



denominator are integers. This may consume a considerable num- 

 ber of time interv^als and allow considerable fluctuation in the tem- 

 perature of the furnace tube. 



An attempt has been made to show this graphically in figure 2. 

 The wavy line represents the excursions of an idealized galvanometer 



