576 



STATE BOARD OF AGRICULTURE. 



Vl<^ of ;in nnipore for 10 second i)eiio(ls. Fij?. 10 is a cut of (he alterna- 

 ting (Minent galvanonietei' used in tliese experiments. 



When a galvanometer of the moving coil type is used, precautions 

 must bo taken against exterior electric fields. All such ])arts as produce 

 this trouble should be incased in metal tubing or placed below the table. 

 Until this was done the galvanometer scale was deflected as much as a 

 scale division. Tliis means an error of .1 of 1%. The data that accom- 

 panied the galvanometer make no mention of its sensitivity except when 

 the stationary coil is so connected as to be in series Avith the swinging 



Fig. 11. 

 Bridge. 



1. Posts for leads to electrolytic cell. 



2. Posts behind drum for galvanometer leads. 



3. Battery post. 



4. Extension wire posts. "Extension" represented as in. 



5. Resistance in bridge base. Disconnected. 



coil. Under this condition the sensitivity is .0027 amperes at 100 mm. 

 Connected up in the other way the sensitivity is probably of the fol- 

 lowing order ,000027 at 100 mm. A double contact switch on the top of 

 the table and near the operator controls the galvanometer and bridge cir- 

 cuits. The circuit can be made and broken as the conditions require. It 

 is never left on for more than a minute and usually only for a second or 

 less. As soon as a deflection is noticed the circuit is broken and an ad- 

 justment of the bridge made. The circuit is again closed and the deflec- 

 tions of the scale observed. This operation is repeated until a balance is 

 obtained. By this means errors from heating and polarization are elimin- 



