A galvanometer maybe converted into an ammeter by connecting a very low resistance shunt in parallel with it.

A galvanometer may be converted into a voltmeter by connecting a very high resistance multiplier in series with it.

With switch S open:

V1 reads the EMF

Ammeters read zero

With switch S closed and no internal resistance:

V₁ reads EMF

A₁ = A₂ + A₃ = A₄

V₁ = V₂ + V₃ + V₄

With switch S closed and internal resistance:

V₁ reads tpd (terminal potential difference)

R_total = R_internal + R_external

R_external = r₁ + r₄ + effective resistance of parallel resistors.

V = IR

EMF = I R_total

Tpd = I Rexternal

Lost volts = EMF – tpd

Lost volts = I R internal

N.B. lost volts increase as the current increases.

Lost volts are the volts used with in the cells to drive the current against the internal resistance.

I_series can be calculated in many ways it depends on what you are given.

I = ^EMF/R_total I = ^{lost volts}/R _internal

I = ^tpd/R_external I = ^Vindividual/R_individual

Refer to circuit on page 3

I = ^V4/_R4 I = ^V2 /_R1 I = ^V3/ _Rparallel

e.g. In the following circuit when switch S is open V₁ reads 16V. When switch S is closed V₁ reads 14V and V₂ reads 2V.

a) Calculate the reading on A.

b) Find the equivalent resistance of the parallel resistors

c) Find the total resistance

d) Find the internal resistance

e) Find the reading on V₃

f) Find the current in the 3W resistor.

a) I = ^V/_R = ^V2/R_individual = ²/₁ = 2A

b) ¹/_R = ¹/r₁ + ¹/r₂ = ¹/₃ + ¹/₆ = ³/₆ R = 2W

c) R_total = ^emf/_I = ¹⁶/₂ = 8W

d) R_internal = R_total - R_external = 8 – 7 = 1W

OR

R internal = ^{lost volts}/ _I = ^{(16 – 14)} /₂ = 1W

e) V₃ =IR = 2 x2 = 4V

f) I = ^V/_R = 4/3 = 1,3 A