Name of primary key column is always id when using Laravel 5.3?

Let say I have a table with table structure as following:

Table_A
    sid (auto increment, primary)
    .... some other column ....
    created_at (created using laravel eloquent timestamps() function)
    updated_at (same as above)

Then, I created a new column by.

$new_row = new TableA; //TableA is a model created pointing to Table_A
$new_row->col1 = 'Some value';
$new_row->col2 = 'Some other value';
// Some other field
$new_row->save()

Then, right after this section of code,

dd($new_row->sid);

Surprisingly, the output is:

null

However, when I change it to:

dd($new_row->id);

It returns the sid (i.e. the primary key value of the row). Is the name of the primary key always id when I am using auto-increment as my primary key in Laravel Model? Or why am I only getting the value when I use the name id?

via Chebli Mohamed