Question

why we don't use high resistance in careyfoster bridge?

Answer 1

To measure a very small amount of resistance, and to increase the sensitivity of the bridge, large resistances are not used in Carey foster bridge.

We can even measure the decimal value of resistance, and hence Carey foster bridge is very sensitive. Say, we have to measure the value of some unknown resistance of about 500 ohms with a meter bridge wire of say 20 ohms Resistance, then the bridge will not be so sensitive. In Carey foster bridge this problem is overcome by taking two standard resistances of the same order in addition so that they in series with the wire make the new ratio arms. Similar arguments can be made for resistances of the order of fraction of ohms.