68 ... Mr. Bollo Appleyard on a 



connected between a and b, is balanced against the 45 ohms 

 between b and e ; the slider has then a position at the middle 

 of the slide-wire. When p is switched to n, the platinum 

 thermometer is substituted for the (b, e) coil. 



Calibration. 

 To calibrate the bridge, the switch (fig. 2) is set with p, g 

 horizontal, and s screwed firmly to m. This puts 45 + 10 = 55 

 ohms between b and e, and the slider has to be moved to a 

 new position along the plank. A simple calculation enables 

 us to find an expression for the temperature, t, of the platinum 

 coils in terms of n, the distance in cms. of the sliding-contact 

 from the extreme left of the plank- scale. The actual figures 

 for this particular bridge are here given. 



Corrected values of the bridge-coils, at 80° F. 



Between a and b = 45*138 ohms. 

 „ c „ e = 45*150 „ 

 „ h „ c = 10-032 „ 



The coils 100, 100 require no correction ; they were ad- 

 justed at 64°-5 F., and we are only concerned with their 

 equality at the slightly higher temperature. 



Value of r, the resistance of 1 cm. of the slide-wire. — 

 Balancing (45*15 + 10*032) ohms against 45*138 and noting 

 the corresponding value* of n, we have 



45*138[100 + 241-95 r]=55-182[100 + 58*05 r]. 



Whence r=«1301 ohm (2) 



Kj in terms of t. — Putting the values for B 32 and a in (1), 

 the equation becomes 



K.,=40-05[l + -002097(£-32)], ... (3) 



that is, within the required range of temperature. 



To this may be added, without sensible error, the resistance 

 of the thick copper leads going to the platinum coils ; this 

 was *025 olim ; so that 



K,= 37-39 + 0-084^ (4) 



Rj in terms of n. — The general equation of the bridge 

 will be 



B,[100+(300-n)r] =45*138(100 +nr). 



Or . 4513*8 + 5*8745 n 



«- 139*04-*1301n W 



* The full length of the slide-wire is rc=300 cms. 



