Abstract: The rotational inertia of complex or heterogeneous rigid body can be measured by trifilar torsional

pendulum.The torsion pendulum period is an important parameter related to the accuracy of the test. Manual

counting and stopwatch timing are very laborious and easy to make mistakes.The photoelectric sensor and the measurement

are easily affected by the yawing or shaking，which will lead to the omission of one period.To this end，a

new type of trifilar torsional pendulum periodic measuring instrument is proposed，which consists of light source，polarizer，

detector，photosensitive resistance module，data acquisition system and computer installed with upper computer

software.The polarization characteristics of light and Marius's law are used to detect the torsion pendulum situation

of the trifilar torsional pendulum.We can get the period of trifilar torsional pendulum by using the linear fitting

function of origin data processing software.Using the photoelectric measurement device in the market and the new

trifilar torsional pendulum period measuring instrument at the same time，a comparative experiment is carried out to

measure the moment of inertia of the lower plate of trifilar torsional pendulum.The experimental results show that the

relative error of the existing photoelectric measuring device is 0.83%，and the relative error of the new trifilar torsional

pendulum period measuring instrument is up to 0.11%， indicating that the accuracy of the new type trifilar

torsional pendulum periodic measuring instrument is higher than the existing photoelectric measuring device.Because

the intensity of the transmission light is not affected by the small range swing or sloshing of the trifilar torsional pendulum，

the new rifilar torsional pendulum periodic measuring instrument has a strong anti-interference ability.Besides，

the device has the advantages of fast measuring speed， simple operation，convenience，and has certain popularization

value.

Key words: rotational inertia, trifilar torsional pendulum, polarization, photosensitive resistance, yaw