ELECTRICAL MEASUREMENTS 183 



rectly in amperes, milliamperes (ma), or microamperes (/-ta). The 

 same meter can be used to measure voltage if an unvarying resistor is 

 connected in series with the coil. Current through the resistor (and the 

 coil) depends directly on the voltage. If the resistor is built into the 

 instrument, the meter scale can be graduated in volts. A dry-cell (or other 

 constant voltage source) and a meter can be used to measure resistance, 

 once again because I = E/R. 



Speakers: The use of speakers as output transducers is sometimes ad- 

 vantageous. In a complex instrumental arrangement, for example, the 

 operator's eyes might be too busy to watch a meter. His ears, then, 

 could detect changes in volume or pitch from the speaker. In some 

 instruments speakers are used chiefly for demonstration purposes, while 

 in others the speaker gives a warning when some misuse of the instru- 

 ment or some other disaster is imminent. Speakers can also be used in 

 conjunction with other output transducers. 



Recording Instruments: The ultimate in convenience comes with the 

 use of a recording instrument as the output transducer. A pen scribes a 

 permanent record on a moving paper. Most frequently used are the vari- 

 ous strip-chart recorders, in which paper from a roll is fed under the 

 pen. The pen moves across the paper by an amount proportional to the 

 strength of the electrical signal. 



The recorder often contains a galvanometer and thus will measure 

 current or, with an appropriate resistor, voltage. A pen is attached to 

 the moving coil of the meter. Variations in the electrical signal are 

 recorded as a curve, varying distances from some zero line. Since the 

 paper moves with constant speed, the curve is plotted as a function of 

 time. 



The potentiometer recorder is more complicated but can respond 

 more reliably to smaller fluctuations. The instrument is a null measuring 

 device because the signal creates an imbalance in the instrument. The 

 instrument responds to this imbalance by actuating a motor which in- 

 creases or decreases the resistance in the circuit, thus restoring the bal- 

 ance. The amount of motor movement required to return the instrument 

 to the balanced condition is recorded on the paper. In several of the in- 

 struments the pen is attached to the same motor by a gear or pulley 

 arrangement. The Brown Electronik Potentiometer Recorder is one of 

 the favored instruments in laboratories everywhere. Several others may 

 be as good or even better for some purposes. 



Oscilloscopes: For relatively fast, high frequency responses, the cath- 

 ode ray oscilloscope is used. This instrument is a vacuum tube in 



