Amplifying the output of a thermistor
We have a XY problem here.

OP says they have a heating element with a sensor, which they believe is a PTC. 

It is not.

After reading the long, long comment thread, I've come to the conclusion that what you actually have is a thermocouple embedded in the sensor. Unlike an NTC or PTC, a thermocouple outputs a voltage proportional to temperature.

How to interface it? The thermocouple outputs in the tens of millivolts range. With this low voltage, what's needed is to use a differential amplifier, using both leads from the sensor. There are dedicated ICs for this, one of which appears in the Arduino project link below.

But there's more besides just amplifying the voltage. Thermocouples also have different characteristics depending on the metal-metal junction they use. A common type is K (Nickel-Chromium / Nickel-Alumel) but there are many others with different output curves. They are not linear, and need to have that corrected to get the right temperature value.

Here's a link to interfacing Arduino to Type K thermocouple: https://www.electronicwings.com/arduino/thermocouple-interfacing-with-arduino-uno

Lots of other ideas can be found - thermocouple interfacing is a popular Arduino project.

[edited away - use NTC and voltage divider]