The length of metallic wire is $l _ { 1 }$ when tension in it is $T _ { 1 }$. It is $l _ { 2 }$ when the tension is $T _ { 2 }$. The original length of the wire will be :\\
(1) $\frac { T _ { 2 } l _ { 1 } + T _ { 1 } l _ { 2 } } { T _ { 1 } + T _ { 2 } }$\\
(2) $\frac { l _ { 1 } + l _ { 2 } } { 2 }$\\
(3) $\frac { T _ { 2 } l _ { 1 } - T _ { 1 } l _ { 2 } } { T _ { 2 } - T _ { 1 } }$\\
(4) $\frac { \mathrm { T } _ { 1 } l _ { 1 } - \mathrm { T } _ { 2 } l _ { 2 } } { \mathrm {~T} _ { 2 } - \mathrm { T } _ { 1 } }$