### Q: What is dark matter and why is it important to our understanding of the universe? - Predrag

Dark matter is matter that is invisible to our normal methods of observing matter in galaxies, yet it still has gravitational effects on visible matter. No one is quite sure what dark matter consists of and it's a topic of a lot of ongoing research.

Dark matter was originally theorized by an astronomer by the name of Fritz Zwicky who saw a large discrepancy between the observed mass of galaxies (the visible matter) and their computed masses (computed from gravitational effects). This discrepancy is known as the "missing mass problem." This missing mass is termed dark matter -- it is "dark" because it's not visually observable.

Understanding dark matter helps us understand the history of the universe. Our current knowledge of the formation of galaxies is not consistent with theories that do not involve dark matter, so the more we know about dark matter, the more we understand how these galaxies originally formed. Also, it helps us understand current behavior of galaxies. Without dark matter, objects further away from the center of a galaxy should move slower, but experimental evidence shows that instead, this motion is constant after a certain radius. Dark matter explains this anomaly.

An important concept related to dark matter is dark energy. Dark energy helps us understand the universe's expansion. More information on dark matter and dark energy can be found here and here.

### COMMON COLLECTOR CONFIGURATION OF A TRANSISTOR

COMMON COLLECTOR CONNECTION

In  this  configuration  the  input  is  applied  between the  base  and  the  collector and  the  output  is  taken  from  the  collector  and  the  emitter.  Here  the  collector  is common to both the input and the output circuits as shown in Fig.

Common Collector Transistor Circuit

In  common  collector  configuration  the  input  current  is  the  base current  IB  and  the output current is the emitter current IE. The ratio of change in emitter current to the  change in the base current is called current amplification factor.

It is represented by

COMMON COLLECTOR CIRCUIT

A test  circuit  for determining the  static characteristic  of an NPN transistor is shown in Fig. In this circuit the collector is common to both the input and the output circuits.   To   measure   the   base   and   the   emitter   currents,   milli   ammeters   are connected in series with the base and the emitter circuits. Voltmeters are connected   across the input an…