Given instrument is called as:
Electronic Method of Determining Working Length (Electronic Apex Locators)
In recent years, electronic devices have been developed for determining the working length of a root canal without resorting to radiography. 
Concept: 

The working length is determined by comparing the electrical impedance of the periodontal membrane with that of the oral mucosa both of which should be similar at 6.5 KΩ. 
This is done with the help of an electronic apex locator cord that has two ends. 
One end is termed as a “lip hook” that is kept in contact with the oral mucosa of the patient, while the other end is termed as “file holder” that is a probe which is attached to an endodontic instrument (K file or rotary file). 
The attached file is slowly inserted into the root canal up to the estimated working length. 
When the endodontic file touches the soft tissues of the periodontal membrane, the electrical-resistance gauges for both oral mucosa and periodontal ligament would have similar readings. 
By measuring the depth of insertion of the endodontic file, one may determine the exact working length of the root canal.
Apexum
Apex locator
Vitalometer
Electronic canal cleaner