////; SI luiiortioxnr .ix r.i.ncri'Hc.ii. sii-.nioscoi't-: 



539 



i>f .ihout :U) '/'(' a\ailal)lu for olit.iiiiiiii; ^rt-atiT inli'iisity of soiiikU or 

 for stipplyiii^ a lar^t- iuinil>or of individual listuiiiiiK units. 



The potentiometer between the lii^t .md second tulies makes it 

 possible to adjust the ampht'ieation in small steps, each step K'^'i'iK 



300 600 lOO 



rBtoutrnCy 

 e-fCLtS PE.8 StCONO 



I'i^;. 5. — Amplifur characteristics — maxiinum amplification 



apprt)ximately twice the energy of the preceding one. This is an 

 essential element of a fle.xible system. 



(■). Ki,[;< IRK I"ii.ti-:rs 



.An electric tiller is a combination of coils and condensers capable of 

 separating electrical waves characterized by a difference in frequency.''' 



The three fundamental forms of filters are commonly termed 

 "low-pass," "high-pass," and "band-pass." A low-pass filter is one 

 which passes currents of frecjuencies below a particular "cut-off 

 frequency" and attenuates or weakens very greath' currents of higher 

 frequencies. A high-pass filter does the opposite — attenuates below 

 the cut-ofT frecjuency and passes above this fretjuency. A band- 

 pass filter is one which passes currents of frequencies within a definite 

 band fi.xed by two cut-off frequencies. A low-pass and a high-pass 

 filter connected in series constitute one form of band-pass filter. 

 For any type of filter, the sharpness of cut-off and the amount of 

 attenuation can be controlled at will by suitable design constants. 



"C. .\. Campbell, "Physical Thcor>- of Electrical Wave Filters," Bell Syslfin 

 Tech. Journal, Nov. l')2i. 



